…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>