This is not quite the same as checking whether tp_getattro
has been updated, since setting a __getattr__ will change
tp_getattro.

Usually this isn't that important since we will still need
to do some special getattr handling, but in a couple specific
cases we can optimize based on it.  In particular, this commit
optimizes calls to isinstance(inst, cls) on instances which have
a __getattr__ set: in the common case (no __getattribute__,
no custom __class__) we can know that the __getattr__ will not
get called and we can stay on the isinstance fastpath.