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/classobject.c

@@ -73,7 +73,7 @@ PyMethod_New(PyObject *func, PyObject *self)
 }
 
 static PyObject *
-method_reduce(PyMethodObject *im)
+method_reduce(PyMethodObject *im, PyObject *Py_UNUSED(ignored))
 {
     PyObject *self = PyMethod_GET_SELF(im);
     PyObject *func = PyMethod_GET_FUNCTION(im);