Default to always run unit tests for both py2 and 3

Doing make test will with this change assume that you have a working python3 environment (hey, why shouldn't you? It's the future, right?). It will first run the tests with python2 and then move on to python3.

If you only want to run the test suite using python2, you can still do `./run_tests`.

I'm not sure it would catch my recent blunder, but with some more test coverage, it might have :-).