PEP 743: Add Py_COMPAT_API_VERSION to the Python C API

Since recommendations change over time, it does feel like there should be a version number related aspect to this.

However, “limit this build to the recommended C API as of the release of Python 3.13” feels like a better way of expressing that than “limit this build to the C API that is expected to still be compatible with Python 3.14” (or 3.15, or whatever).

Opting in to the recommended API for a release would not only exclude outright deprecated functions, but also a new category of “discouraged” functions that we have no plans to remove, but folks should still avoid using in new code.

(Note: the advantage of using a future release version in the macro is that it means deprecations can continue to only specify the release where they’re going away. For a “recommended” API flag to work, deprecated APIs would also need to be marked as discouraged in the release where they first became deprecated)