property_traverse() should also traverse into prop_doc -- there's no

typecheck that guarantees it's a string, and BTW string subclasses
could hide references.

Objects/descrobject.c

@@ -1201,6 +1201,7 @@ property_traverse(PyObject *self, visitproc visit, void *arg)
 	VISIT(prop_get);
 	VISIT(prop_set);
 	VISIT(prop_del);
+	VISIT(prop_doc);
 
 	return 0;
 }