gh-144175: Add PyArg_ParseVector() function

[vstinner]

• Issue: Add public PyArg_ParseTupleAndKeywords API equivalent for METH_FASTCALL #144175

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📚 Documentation preview 📚: https://cpython-previews--144283.org.readthedocs.build/

[vstinner]

The change doesn't include tests because my plan is to write a following PR to rename _PyArg_ParseStack() to PyArg_ParseVector(). Many modules using Argument Clinic are calling _PyArg_ParseStack(). I prefer to write a separated PR since it changes a lot of code and so is harder to review.

[vstinner]

Ok, I completed the PR to add also PyArg_ParseVectorAndKeywords() function. The PR is now ready for review :-)

[vstinner]

I propose the following API:

int PyArg_ParseVector(
    PyObject *const *args,
    Py_ssize_t nargs,
    const char *format,
    ...);

int PyArg_ParseVectorAndKeywords(
    PyObject *const *args,
    Py_ssize_t nargs,
    PyObject *kwnames,
    const char *format,
    char * const *kwlist,
    ...);

[picnixz]

Any reason to have PY_CXX_CONST? why now always const char *const *kwlist? (are there occurrences where we don't have a const?)