bpo-47164: Argument Clinic uses _PyCFunction_CAST()

[vstinner]

Replace "(PyCFunction)(void(*)(void))func" cast with
_PyCFunction_CAST(func).

https://bugs.python.org/issue47164

[arhadthedev]

What to do with yet unmerged PRs that add Argument Clinic stuff (like #32010)? They'll introduce more (PyCFunction)(void(*)(void)).

[vstinner]

You should ignore this draft PR for now. I'm still thinking about it :-) If there will be a conflict, I will simply regenerate this PR (rerun make clinic).

[vstinner]

I rebased my PR.

[vstinner]

Example of change:

-    {"test_object_converter", (PyCFunction)(void(*)(void))test_object_converter, METH_FASTCALL, test_object_converter__doc__},
+    {"test_object_converter", _PyCFunction_CAST(test_object_converter), METH_FASTCALL, test_object_converter__doc__},