Add _Py_PACK_VERSION for CPython's own definitions Py_PACK_VERSION was added to limited API in 3.14, so if Py_LIMITED_API is lower, the macro can't be used. Add a private version that can be used in CPython headers for checks like `Py_LIMITED_API+0 >= _Py_PACK_VERSION(3, 14)`.
Miscellaneous source files for the main Python shared library