Change in policy: when a slot_tp_xxx function looks for the __xxx__ method,

Change in policy: when a slot_tp_xxx function looks for the __xxx__ method,
don't use getattr, but only look in the dict of the type and base types.
This prevents picking up all sorts of weird stuff, including things defined
by the metaclass when the object is a class (type).

For this purpose, a helper function lookup_method() was added.  One or two
other places also use this.