gh-91320: Use _PyCFunction_CAST() (#92251)

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

Change generated by the command:

sed -i -e \
  's!(PyCFunction)(void(\*)(void)) *\([A-Za-z0-9_]\+\)!_PyCFunction_CAST(\1)!g' \
  $(find -name "*.c")