[Python-3000] Traits/roles instead of ABCs

[Bill Janssen]

> To me, interfaces and/or generic functions strike the right balance.

I agree.  As I've said before, if this was 1994 I think I'd be in the
PJE camp and prefer generic functions.  As it is, I think interfaces
better fit the current state of Python.  And I think the existing type
system has everything that's needed to indicate interfaces.  All we
need are some base definitions to stand on (dict, sequence, file, etc.).

> Such tools are completely invisible for Python programmers who don't =20
> care about them (the vast majority).  They're also essential for a =20
> very small subclass of very important Python applications.

Yep, those of us who write very large Python applications.

> If ABCs can walk that same tightrope of utility and invisibility, =20
> then maybe they'll successfully fill that niche.

I'm sure they will.

Bill