Fix __slots__ definitions

According to http://docs.python.org/reference/datamodel.html#slots



* The action of a __slots__ declaration is limited to the class where it
  is defined. As a result, subclasses will have a __dict__ unless they
  also define __slots__ (which must only contain names of any
  /additional/ slots).

* If a class defines a slot also defined in a base class, the instance
  variable defined by the base class slot is inaccessible (except by
  retrieving its descriptor directly from the base class). This renders
  the meaning of the program undefined. In the future, a check may be
  added to prevent this.

Signed-off-by: Balazs Lecz <leczb@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>
Signed-off-by: Iustin Pop <iustin@google.com>