pyo3-ffi: expose PyObject_Vectorcall(Method) on the stable abi on 3.12+

[Icxolu]

This exposes PyObject_Vectorcall(Method) on the stable abi on Python 3.12+ (extracted from #4379, with minor modification on PY_VECTORCALL_ARGUMENTS_OFFSET cfgs) and enables some of our vectorcall optimizations on the limited api.

[davidhewitt]

Thanks for extracting this out; I will try to rebase #4379 after this merges and assess how much is left / still needs to be split from there.

LGTM with one suggestion for a test...

[Icxolu]

Hmm, looks like something does not work for Python 3.9 and 3.10, due to "undefined reference to PyObject_Vectorcall" (and on macos _PyObject_Vectorcall for some reason). Currently I'm not sure why. According to the docs it should be available from Python 3.9 and from 3.12 in the limited api.

[davidhewitt]

Ugh, I think it was only exposed as a symbol from 3.11: python/cpython#28893

Before then it was a static inline function so the existing definition we had in cpython/abstract.rs was correct. We probably need to restore some of that code and adjust the cfgs.

(I guess this was my error from the original PR, sorry about that!)

[Icxolu]

Thanks for the hint! I adjusted the cfgs, lets see if this works.

[davidhewitt]

Hooray! 🙌