gh-111178: Fix PyCMethod API

[vstinner]

Replace "size_t nargs" with "Py_ssize_t nargs" in PyCMethod.

• Issue: UBSan: Calling a function through pointer to incorrect function type is undefined behavior #111178

[vstinner]

PyCMethod is documented and the documentation already uses Py_ssize_t: https://docs.python.org/dev/c-api/structures.html#c.PyCMethod

[vstinner]

@encukou: Is it ok to change the sign of the nargs parameter in PyCMethod? In practice, Python already calls PyCMethod with signed Py_ssize_t and functions are already implemented with signed Py_ssize_t. Also, as I wrote, the documentation also already uses signed Py_ssize_t.

[encukou]

I believe it is. Thanks!