gh-111178: Change Argument Clinic signature for METH_METHOD

[vstinner]

Replace "Py_ssize_t nargs" with "size_t nargs" for METH_METHOD|METH_FASTCALL functions to fix an undefined behavior.

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

[vstinner]

Oh, there is a new compiler warning:

In file included from ../cpython-ro-srcdir/Modules/_io/clinic/bufferedio.c.h:11,
                 from ../cpython-ro-srcdir/Modules/_io/bufferedio.c:2505:

../cpython-ro-srcdir/Modules/_io/clinic/bufferedio.c.h: In function ‘_io__BufferedIOBase_read’:
Warning: ../cpython-ro-srcdir/Include/internal/pycore_modsupport.h:90:16: warning: comparison of unsigned expression in ‘>= 0’ is always true [-Wtype-limits]
   90 |       (minpos) <= (nargs) && ((varpos) || (nargs) <= (maxpos)) && (args) != NULL) ? \
      |                ^~

../cpython-ro-srcdir/Modules/_io/clinic/bufferedio.c.h:154:12: note: in expansion of macro ‘_PyArg_UnpackKeywords’
  154 |     args = _PyArg_UnpackKeywords(args, nargs, NULL, kwnames, &_parser,
      |            ^~~~~~~~~~~~~~~~~~~~~

[vstinner]

I abandon this PR in favor of #131135.