convert py3 tests to py2 compatible versions

[mr-c]

• [ ] enable py3 & py3 running of the tests in .travis.yml (possible use tox to manage python versions?)
• [ ] don't use Mock if passing in arguments directly to argparse is sufficient
• [ ] for remaining cases just monkeypatch the library as needed See https://github.com/erasche/argparse2tool/pull/32#issuecomment-260562033

Anton, if you don't have time for this soonish let me know and I'll do it.