Message3422

Currently, almost every lookup on a new-style instance (*Derived) does
in fact two lookups: first finds the '__getattribute__' slot in the type
mro, then the actual attribute lookup takes place. And, if
__getattribute__ raises AttributeError,  another mro lookup takes place,
this time for __getattr__.

This is somewhat solved on old-style instances (PyInstance), where the
__getattr__ slot is cached on PyClass. Although it has a problem: the
cache is not invalidated/refreshed when the __getattr__ slot is
dinamically changed.