[MERGE] Make the test suite run under Python 2.7a0

[Andrew Bennetts]

John Arbash Meinel wrote:
[...]
> 
> BB:approve
> 
> Though I'll comment that you could have left TestCase.exc_info() and
> just had it return sys.exc_info().

I could have, but what for?  Do you think users of bzrlib were using
TestCase.exc_info directly?  It seemed reasonable to me to just delete the
unnecessary cruft; it was always equivalent to calling sys.exc_info()
anyway.

Thanks for the review.  I've submitted the branch as-is; we can always
restore TestCase.exc_info() before 1.14 if it turns out there's a reason to
do so.

-Andrew.