[Python-3000] PEP 3134 exception reporting

[Antoine Pitrou]

Hi,

I've submitted a final patch with the following variations:
 - no additions or changes to public APIs
 - tracebacks are standardized as suggested by Georg Brandl
 - reference cycles along the __context__ chain are broken in PyErr_SetObject
(it is an O(n) linear search but should show almost zero overhead in the 
common case where context chains are trivially short)

Regards

Antoine.