Availability of PyMutex to implement locking for thread-safety

[rgommers]

Relevant discussions for making PyMutex public in CPython for 3.13:

• https://discuss.python.org/t/make-some-pymutex-apis-public/50203
• Make some PyMutex functions public python/cpython#117511

The second link notes some current alternatives, like PyThreadType_lock or platform-specific locks.

[ngoldbaum]

The C API working group decided to expose PyMutex in the C API capi-workgroup/decisions#22 (comment). We’re after the code freeze so it will take a special decision from the release manager for it to be included in 3.13 but it will definitely be in 3.14.

[colesbury]

PyMutex, PyMutex_Lock(), and PyMutex_Unlock() are now public in 3.13 (will be available in beta 3).

[colesbury]

Also the critical section API (i.e., Py_BEGIN_CRITICAL_SECTION() is now available publicly)

[rgommers]

This is all done and documented in https://py-free-threading.github.io/porting/ now, so let's close this.