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/object.c

@@ -1619,13 +1619,13 @@ NotImplemented_repr(PyObject *op)
 }
 
 static PyObject *
-NotImplemented_reduce(PyObject *op)
+NotImplemented_reduce(PyObject *op, PyObject *Py_UNUSED(ignored))
 {
     return PyUnicode_FromString("NotImplemented");
 }
 
 static PyMethodDef notimplemented_methods[] = {
-    {"__reduce__", (PyCFunction)NotImplemented_reduce, METH_NOARGS, NULL},
+    {"__reduce__", NotImplemented_reduce, METH_NOARGS, NULL},
     {NULL, NULL}
 };