…related issues (pythonGH-120520)* Add an InternalDocs file describing how interning should work and how to use it.* Add internal functions to *explicitly* request what kind of interning is done:  - `_PyUnicode_InternMortal`  - `_PyUnicode_InternImmortal`  - `_PyUnicode_InternStatic`* Switch uses of `PyUnicode_InternInPlace` to those.* Disallow using `_Py_SetImmortal` on strings directly.  You should use `_PyUnicode_InternImmortal` instead:  - Strings should be interned before immortalization, otherwise you're possibly    interning a immortalizing copy.  - `_Py_SetImmortal` doesn't handle the `SSTATE_INTERNED_MORTAL` to    `SSTATE_INTERNED_IMMORTAL` update, and those flags can't be changed in    backports, as they are now part of public API and version-specific ABI.* Add private `_only_immortal` argument for `sys.getunicodeinternedsize`, used in refleak test machinery.* Make sure the statically allocated string singletons are unique. This means these sets are now disjoint:  - `_Py_ID`  - `_Py_STR` (including the empty string)  - one-character latin-1 singletons  Now, when you intern a singleton, that exact singleton will be interned.* Add a `_Py_LATIN1_CHR` macro, use it instead of `_Py_ID`/`_Py_STR` for one-character latin-1 singletons everywhere (including Clinic).* Intern `_Py_STR` singletons at startup.* For free-threaded builds, intern `_Py_LATIN1_CHR` singletons at startup.* Beef up the tests. Cover internal details (marked with `@cpython_only`).* Add lots of assertionsCo-Authored-By: Eric Snow <ericsnowcurrently@gmail.com>

…d issues (GH-120520) (GH-120945)* Add an InternalDocs file describing how interning should work and how to use it.* Add internal functions to *explicitly* request what kind of interning is done:  - `_PyUnicode_InternMortal`  - `_PyUnicode_InternImmortal`  - `_PyUnicode_InternStatic`* Switch uses of `PyUnicode_InternInPlace` to those.* Disallow using `_Py_SetImmortal` on strings directly.  You should use `_PyUnicode_InternImmortal` instead:  - Strings should be interned before immortalization, otherwise you're possibly    interning a immortalizing copy.  - `_Py_SetImmortal` doesn't handle the `SSTATE_INTERNED_MORTAL` to    `SSTATE_INTERNED_IMMORTAL` update, and those flags can't be changed in    backports, as they are now part of public API and version-specific ABI.* Add private `_only_immortal` argument for `sys.getunicodeinternedsize`, used in refleak test machinery.* Make sure the statically allocated string singletons are unique. This means these sets are now disjoint:  - `_Py_ID`  - `_Py_STR` (including the empty string)  - one-character latin-1 singletons  Now, when you intern a singleton, that exact singleton will be interned.* Add a `_Py_LATIN1_CHR` macro, use it instead of `_Py_ID`/`_Py_STR` for one-character latin-1 singletons everywhere (including Clinic).* Intern `_Py_STR` singletons at startup.* For free-threaded builds, intern `_Py_LATIN1_CHR` singletons at startup.* Beef up the tests. Cover internal details (marked with `@cpython_only`).* Add lots of assertionsCo-authored-by: Eric Snow <ericsnowcurrently@gmail.com>

… issues (pythonGH-120520)* Add an InternalDocs file describing how interning should work and how to use it.* Add internal functions to *explicitly* request what kind of interning is done:  - `_PyUnicode_InternMortal`  - `_PyUnicode_InternImmortal`  - `_PyUnicode_InternStatic`* Switch uses of `PyUnicode_InternInPlace` to those.* Disallow using `_Py_SetImmortal` on strings directly.  You should use `_PyUnicode_InternImmortal` instead:  - Strings should be interned before immortalization, otherwise you're possibly    interning a immortalizing copy.  - `_Py_SetImmortal` doesn't handle the `SSTATE_INTERNED_MORTAL` to    `SSTATE_INTERNED_IMMORTAL` update, and those flags can't be changed in    backports, as they are now part of public API and version-specific ABI.* Add private `_only_immortal` argument for `sys.getunicodeinternedsize`, used in refleak test machinery.* Make sure the statically allocated string singletons are unique. This means these sets are now disjoint:  - `_Py_ID`  - `_Py_STR` (including the empty string)  - one-character latin-1 singletons  Now, when you intern a singleton, that exact singleton will be interned.* Add a `_Py_LATIN1_CHR` macro, use it instead of `_Py_ID`/`_Py_STR` for one-character latin-1 singletons everywhere (including Clinic).* Intern `_Py_STR` singletons at startup.* For free-threaded builds, intern `_Py_LATIN1_CHR` singletons at startup.* Beef up the tests. Cover internal details (marked with `@cpython_only`).* Add lots of assertionsCo-Authored-By: Eric Snow <ericsnowcurrently@gmail.com>

… issues (pythonGH-120520)* Add an InternalDocs file describing how interning should work and how to use it.* Add internal functions to *explicitly* request what kind of interning is done:  - `_PyUnicode_InternMortal`  - `_PyUnicode_InternImmortal`  - `_PyUnicode_InternStatic`* Switch uses of `PyUnicode_InternInPlace` to those.* Disallow using `_Py_SetImmortal` on strings directly.  You should use `_PyUnicode_InternImmortal` instead:  - Strings should be interned before immortalization, otherwise you're possibly    interning a immortalizing copy.  - `_Py_SetImmortal` doesn't handle the `SSTATE_INTERNED_MORTAL` to    `SSTATE_INTERNED_IMMORTAL` update, and those flags can't be changed in    backports, as they are now part of public API and version-specific ABI.* Add private `_only_immortal` argument for `sys.getunicodeinternedsize`, used in refleak test machinery.* Make sure the statically allocated string singletons are unique. This means these sets are now disjoint:  - `_Py_ID`  - `_Py_STR` (including the empty string)  - one-character latin-1 singletons  Now, when you intern a singleton, that exact singleton will be interned.* Add a `_Py_LATIN1_CHR` macro, use it instead of `_Py_ID`/`_Py_STR` for one-character latin-1 singletons everywhere (including Clinic).* Intern `_Py_STR` singletons at startup.* For free-threaded builds, intern `_Py_LATIN1_CHR` singletons at startup.* Beef up the tests. Cover internal details (marked with `@cpython_only`).* Add lots of assertionsCo-Authored-By: Eric Snow <ericsnowcurrently@gmail.com>

… issues (pythonGH-120520)* Add an InternalDocs file describing how interning should work and how to use it.* Add internal functions to *explicitly* request what kind of interning is done:  - `_PyUnicode_InternMortal`  - `_PyUnicode_InternImmortal`  - `_PyUnicode_InternStatic`* Switch uses of `PyUnicode_InternInPlace` to those.* Disallow using `_Py_SetImmortal` on strings directly.  You should use `_PyUnicode_InternImmortal` instead:  - Strings should be interned before immortalization, otherwise you're possibly    interning a immortalizing copy.  - `_Py_SetImmortal` doesn't handle the `SSTATE_INTERNED_MORTAL` to    `SSTATE_INTERNED_IMMORTAL` update, and those flags can't be changed in    backports, as they are now part of public API and version-specific ABI.* Add private `_only_immortal` argument for `sys.getunicodeinternedsize`, used in refleak test machinery.* Make sure the statically allocated string singletons are unique. This means these sets are now disjoint:  - `_Py_ID`  - `_Py_STR` (including the empty string)  - one-character latin-1 singletons  Now, when you intern a singleton, that exact singleton will be interned.* Add a `_Py_LATIN1_CHR` macro, use it instead of `_Py_ID`/`_Py_STR` for one-character latin-1 singletons everywhere (including Clinic).* Intern `_Py_STR` singletons at startup.* For free-threaded builds, intern `_Py_LATIN1_CHR` singletons at startup.* Beef up the tests. Cover internal details (marked with `@cpython_only`).* Add lots of assertionsCo-Authored-By: Eric Snow <ericsnowcurrently@gmail.com>

…related issues (pythonGH-120520)* Add an InternalDocs file describing how interning should work and how to use it.* Add internal functions to *explicitly* request what kind of interning is done:  - `_PyUnicode_InternMortal`  - `_PyUnicode_InternImmortal`  - `_PyUnicode_InternStatic`* Switch uses of `PyUnicode_InternInPlace` to those.* Disallow using `_Py_SetImmortal` on strings directly.  You should use `_PyUnicode_InternImmortal` instead:  - Strings should be interned before immortalization, otherwise you're possibly    interning a immortalizing copy.  - `_Py_SetImmortal` doesn't handle the `SSTATE_INTERNED_MORTAL` to    `SSTATE_INTERNED_IMMORTAL` update, and those flags can't be changed in    backports, as they are now part of public API and version-specific ABI.* Add private `_only_immortal` argument for `sys.getunicodeinternedsize`, used in refleak test machinery.* Make sure the statically allocated string singletons are unique. This means these sets are now disjoint:  - `_Py_ID`  - `_Py_STR` (including the empty string)  - one-character latin-1 singletons  Now, when you intern a singleton, that exact singleton will be interned.* Add a `_Py_LATIN1_CHR` macro, use it instead of `_Py_ID`/`_Py_STR` for one-character latin-1 singletons everywhere (including Clinic).* Intern `_Py_STR` singletons at startup.* Beef up the tests. Cover internal details (marked with `@cpython_only`).* Add lots of assertionsCo-authored-by: Eric Snow <ericsnowcurrently@gmail.com>

…H-121903,GH-122303) (#123065)This backports several PRs forgh-113993, making interned strings mortal so they can be garbage-collected when no longer needed.* Allow interned strings to be mortal, and fix related issues (GH-120520)  * Add an InternalDocs file describing how interning should work and how to use it.  * Add internal functions to *explicitly* request what kind of interning is done:    - `_PyUnicode_InternMortal`    - `_PyUnicode_InternImmortal`    - `_PyUnicode_InternStatic`  * Switch uses of `PyUnicode_InternInPlace` to those.  * Disallow using `_Py_SetImmortal` on strings directly.    You should use `_PyUnicode_InternImmortal` instead:    - Strings should be interned before immortalization, otherwise you're possibly      interning a immortalizing copy.    - `_Py_SetImmortal` doesn't handle the `SSTATE_INTERNED_MORTAL` to      `SSTATE_INTERNED_IMMORTAL` update, and those flags can't be changed in      backports, as they are now part of public API and version-specific ABI.  * Add private `_only_immortal` argument for `sys.getunicodeinternedsize`, used in refleak test machinery.   Make sure the statically allocated string singletons are unique. This means these sets are now disjoint:    - `_Py_ID`    - `_Py_STR` (including the empty string)    - one-character latin-1 singletons    Now, when you intern a singleton, that exact singleton will be interned.  * Add a `_Py_LATIN1_CHR` macro, use it instead of `_Py_ID`/`_Py_STR` for one-character latin-1 singletons everywhere (including Clinic).  * Intern `_Py_STR` singletons at startup.  * Beef up the tests. Cover internal details (marked with `@cpython_only`).  * Add lots of assertions* Don't immortalize in PyUnicode_InternInPlace; keep immortalizing in other API (GH-121364)  * Switch PyUnicode_InternInPlace to _PyUnicode_InternMortal, clarify docs  * Document immortality in some functions that take `const char *`  This is PyUnicode_InternFromString;  PyDict_SetItemString, PyObject_SetAttrString;  PyObject_DelAttrString; PyUnicode_InternFromString;  and the PyModule_Add convenience functions.  Always point out a non-immortalizing alternative.  * Don't immortalize user-provided attr names in _ctypes* Immortalize names in code objects to avoid crash (GH-121903)* Intern latin-1 one-byte strings at startup (GH-122303)There are some 3.12-specific changes, mainly to allow statically allocated strings in deepfreeze. (In 3.13, deepfreeze switched to the general `_Py_ID`/`_Py_STR`.)Co-authored-by: Eric Snow <ericsnowcurrently@gmail.com>