[Python-3000] PEP 3134 exception reporting

[Antoine Pitrou]

I would like to say that the patch is ready for review here:
http://codereview.appspot.com/2448

And the corresponding issue: http://bugs.python.org/issue3112

It's probably a bit short for inclusion in beta2, although it would be nice to
exercise it in the wild before it's too late... Right now we don't know whether
the new chained tracebacks will appear useful in practice, and whether long
traceback chains will be frequent or not (although intuitively they shouldn't).

Regards

Antoine.