Python 3 - modernize exception catching syntax.

Old-style syntax:  `except Exception, e:`
New-style syntax:  `except Exception as e:`

These two statements are identical, except that the former has
been deprecated for may versions, and was removed in Python 3.

This converts everything to use the new syntax (which also works
in Python 2).  I had to convert unittest2 as well.  What we really
need to do is just delete unittest2, and use unittest instead since
it is a standard module that ships with every Python distribution.
But this is the path of least resistance for now, although at
some point we will really need to do it.

llvm-svn: 251968