bpo-33012: Fix invalid function cast warnings with gcc 8 for METH_NOARGS. (GH-6030)

METH_NOARGS functions need only a single argument but they are cast
into a PyCFunction, which takes two arguments.  This triggers an
invalid function cast warning in gcc8 due to the argument mismatch.
Fix this by adding a dummy unused argument.

Objects/methodobject.c

@@ -101,7 +101,7 @@ meth_dealloc(PyCFunctionObject *m)
 }
 
 static PyObject *
-meth_reduce(PyCFunctionObject *m)
+meth_reduce(PyCFunctionObject *m, PyObject *Py_UNUSED(ignored))
 {
     PyObject *builtins;
     PyObject *getattr;