[Python-3000] two things

[Andrew Koenig]

> "contract" is a better term, IMO, since it's already used in CS (as in
> Eiffel), and describes the situation more correctly: *behavior* rather
> than *signature*.
> "ability" just doesn't seem right to me: my class is not *able* to be a
> set,
> it *behaves* like a set. it follows the set contract, not "ability"

I avoided suggesting the word "contract" instead of "ability" because I do
not want people to think that the notion is somehow connected to Eiffel
contracts.  As Guido (I think) pointed out, it's closer to Haskell
typeclasses.