That sounds like the right thing to do. You should be able to recognize pure Python __setattr__ implementations by the presence of slot_tp_setattro in the tp_setattro slot. (Likewise for __delattr__.)

It's been really long since I looked at this -- do we still only support single inheritance at the C level? Because otherwise there would be another backdoor.

There's also tp_setattr (no 'o') but I think that's basically unused, and we ignored that in 2003 so we should be able to ignore it now. :-)