Don't try to access __slots__ during copy()

When subclassing Django will copy managers from the ancestor class. Copying in turn calls `copy.copy()` which checks whether the object uses `__slots__` or `__dict__`. This could result in the manager calling `self.get_queryset()` before all models get registered with the ORM.
Merge pull request #105 from patrys/patch-1

Don't try to access __slots__ during copy()