[Python-3000] Abilities / Interfaces

[Andrew Koenig]

> More of a tagging approach, then?
> 
> Something like
> 
>   class MyNewClass (ExistingClass,
> OtherInterfaceIJustNoticedExistingClassImplements):
>     pass
> 
> ?

Maybe, maybe not.  I'm not sure.  I'm thinking that it may be useful to be
able somehow to assert that pre-existing class C has pre-existing ability A,
because the authors of C and A weren't talking to each other at the time
they wrote them.