* However, since tp_subclasses started being necessary
 * to invalidate caches (e.g. by PyType_Modified), that clearing has created
 * a bug.  If the weakref to the subclass is cleared before a finalizer is
 * called, the cache may not be correctly invalidated.  That can lead to
 * segfaults since the caches can refer to deallocated objects (GH-135552
 * is an example).