[Python-3000] Traits/roles instead of ABCs

[Bill Janssen]

> No existing mechanism would allow PEP 3141 to be distributed as a
> third-party module and still impact the built-in types; that's what
> this whole business of __isinstance__ and __issubclass__ is about.

Perhaps 3143 should not be distributed as a third-party module.
Perhaps it should be built into the base Python.

I think this "end-of-April" deadline, and Guido's ABC PEP, has spurred
some useful discussion.  I don't see why it has to cripple Python 3000
by ending that discussion prematurely.

Bill