Make unittest2 support Python 2 and Python 3 with single code base

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Right now you have to install unittest2 for Python 2 and unittest2py3k for 
Python 3.

This is a little clunky when using pip requirements files and tox because you 
have to have two separate requirements files and two separate tox targets.

It would be nice if there were one package that supported both.

I am starting a branch here with changes to support this:

https://bitbucket.org/msabramo/unittest2/branch/py2_and_py3_single_source

Original issue reported on code.google.com by [email protected] on 6 May 2014 at 5:49