Message 392403 - Python tracker

> Is this always the case? If so, can we turn the check in check_set_special_type_attr() into an assert? In any case, Py_TPFLAGS_IMMUTABLETYPE should be used, not !Py_TPFLAGS_HEAPTYPE.

I don't know. In case of doubt, I suggest to only replace !Py_TPFLAGS_HEAPTYPE with Py_TPFLAGS_IMMUTABLETYPE.