[Python-3000] Abilities / Interfaces

[Andrew Koenig]

> Both 'ability' and 'interface' imply (to me, anyway) that the class
> being inspected is an actor, that it 'does something' rather than being
> operated on.

I chose 'ability' because to me it doesn't require that the class being
inspected is active by itself.  For example, it feels natural to me to speak
of a class as "having the totally ordered ability".

The problem I have with 'feature' is that it's already in widespread use
without a formal meaning.