gh-107249: Implement Py_UNUSED() for MSVC (#107250)

Fix warnings C4100 in Py_UNUSED() when Python is built with "cl /W4".

Example with this function included by Python.h:

    static inline unsigned int
    PyUnicode_IS_READY(PyObject* Py_UNUSED(op))
    { return 1; }

Without this change, building a C program with "cl /W4" which just
includes Python.h emits the warning:

    Include\cpython/unicodeobject.h(199):
    warning C4100: '_unused_op': unreferenced formal parameter

This change fix this warning.