bpo-47164: Add _PyCFunctionObject_CAST() macr (GH-32190)

Add _PyCFunctionObject_CAST() and _PyCMethodObject_CAST() macros to
make macros casting their argument easier to read, but also to check
the type of their input in debug mode: assert(PyCFunction_Check(func)
and assert(PyCMethod_Check(func).

Reformat also PyCFunction_XXX() macros for readability.