The previous checkin to clear __slots__ variables did a little bit of the work each time it found another base class.  All the work is contiguous, so we might as well do it all at once at the end.
The previous checkin to clear __slots__ variables did a little bit of
the work each time it found another base class.  All the work is
contiguous, so we might as well do it all at once at the end.

sk-/python2.7-type-annotator,sk-/python2.7-type-annotator,sk-/python2.7-type-annotator