I didn't realize that the traceback was attached to the exception object 
in py3k.  This makes the use of such objects more dangerous because of the  
circular references.  The recommended practice of
exc_type, exc_obj = sys.exc_info()[:2]
is precisely to avoid any "accidents" with the tracebacks.
See also http://mail.python.org/pipermail/python-dev/2005-
August/055251.html
So, can one just clear the __traceback__ member?  That would make sense 
for the unittests, but it would make this manifestation of a GC bug go 
into hiding again.