cc@Rogdham (can't request review).

Follows on from#133670. Again best reviewed commit-by-commit. We remove around 100 lines of C code overall.

The last commit (6a3a135 (#133775)) only serves to merge two functions and contains no behaviour change, but does increase the diff size.

• Rename type specs to add underscores after 'zstd'
• Replace theadd_type_to_module() function with anADD_TYPE macro, usingPyModule_AddType instead ofPyModule_AddObjectRef.
• Remove the unused 'empty_readonly_memoryview', 'str_flush', 'str_read', 'str_readinto', and 'str_write' module state members
• Set the CONTINUE/FLUSH_* constants in Python rather than C. To achieve this, export the three ZSTD_EndDirective enum members from the_zstd module.

• Issue:Implement PEP 784 - Adding Zstandard to the Python standard library #132983