PEP 667 follow-up: hard deprecate PyEval_GetLocals()

[ncoghlan]

During the lead up to the 3.13.0 release, the SC approved an amendment to PEP 667 that escalates Python 3.13's soft deprecation for PyEval_GetLocals() to a hard deprecation in 3.14 for eventual removal in 3.16 (the difficulties of keeping that API working in the face of the other implementation changes made the fundamental flaws in the way this API works more self-evident).

This ticket covers adding the hard deprecation to the C code and the expected removal version to the C API docs.

[gpshead]

(I assigned this to mark based on the comment pointing here but maybe this one isn't for mark specifically, feel free to un/reassign)