gh-65210: Add const qualifiers in PyArg_VaParseTupleAndKeywords() (GH-105958)

Change the declaration of the keywords parameter in functions
PyArg_ParseTupleAndKeywords() and PyArg_VaParseTupleAndKeywords() from `char **`
to `char * const *` in C and `const char * const *` in C++.

It makes these functions compatible with argument of type `const char * const *`,
`const char **` or `char * const *` in C++ and `char * const *` in C
without explicit type cast.

Co-authored-by: C.A.M. Gerlach <CAM.Gerlach@Gerlach.CAM>