@chriddyp @cldougl @Kully, any objections?

^^ still working on ensuring the base python versions line up correctly. not sure why that's not better documented, pretty important!

Man. There's always something... I decided to freeze some deps into our tox tests, which messed up support for Python 3.3 (pandas doesn't seem to like being installed on Python 3.3, I should be able to knock it to core-support-only for Python3.3...).

Also, for whatever reason, accessing 3.5 on Circle CI via environment variables is super painful... I ended up just being absurdly specific about it so that tox/circle don't do any guessing about finding Python interpreters, since they were being bad guessers...

Woo! Aight, I think we're good to go! I'm shocked that our coverage is above 70% (not too bad, folks!)

I just added one more change to make sure we're explicitly printing out the python version that's being tested (as a sanity check). If that all passes, I'll likely go ahead and merge this in since it only really deals with how tests are run and doesn't change code other than that.

They're not extensive, but I did write up some documentation in contributing.md that might be helpful for understanding our tox setup. It's actually incredibly flexible and while it wasn't the easiest to set up, I think we'll start to profit as we decide we need/want to support more complicated environments in our test suites 🍻.

//cc @chriddyp @Kully @cldougl

💃 Looks good!