gh-128629: Add _Py_PACK_VERSION for CPython's own definitions (GH-134247)

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)`.