Suggest clarification on behaviour of the __slots__ attribute when
inheriting from classes that don't have __slots__ defined. Obviously the
superclass automatically creates __dict__, and it seems the subclass
inherits this. I presume this is expected behaviour, but I think it
would be worth clarifying in the 'Notes on using __slots__' section -
perhaps add something like:

"If you define __slots__ on a subclass when its superclass doesn't have
__slots__ defined, the superclass will automatically create a __dict__
instance which will be inherited by the subclass (as will other instance
attributes). Defining __slots__ on the subclass doesn't block this
inheritance."

Added a note in r65136, thanks.