Issue: 2354: Add 3K warning for the cmp argument to list.sort() and sorted().

Objects/listobject.c

@@ -2037,6 +2037,11 @@ listsort(PyListObject *self, PyObject *args, PyObject *kwds)
 	}
 	if (compare == Py_None)
 		compare = NULL;
+	if (compare == NULL && 
+            Py_Py3kWarningFlag &&
+	    PyErr_Warn(PyExc_DeprecationWarning, 
+		       "In 3.x, the cmp argument is no longer supported.") < 0)
+		return NULL;
 	if (keyfunc == Py_None)
 		keyfunc = NULL;
 	if (compare != NULL && keyfunc != NULL) {