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/namespaceobject.c

@@ -173,7 +173,7 @@ namespace_richcompare(PyObject *self, PyObject *other, int op)
 PyDoc_STRVAR(namespace_reduce__doc__, "Return state information for pickling");
 
 static PyObject *
-namespace_reduce(_PyNamespaceObject *ns)
+namespace_reduce(_PyNamespaceObject *ns, PyObject *Py_UNUSED(ignored))
 {
     PyObject *result, *args = PyTuple_New(0);