FR: Allow private runtime config to enable extending without breaking the `PyConfig` ABI

[ronaldoussoren]

@vstinner

In 2019, when PyConfig API was designed, I proposed to store the structure size in the structure to make it future-proof in terms of ABI. Bu this idea got rejected: Mailman 3 PEP 587 (Python Initialization Configuration) updated to be future proof again - Python-Dev - python.org In short, the only use case is about embedding Python, and for this use case, it’s ok to require rebuilding Python.

I don’t agree. For tools like py2app/py2exe/pyinstaller it it is pretty inconvenient to have to rebuild the launcher executable that’s used to start the packaged application when there’s a bug fix release of Python.