TST: Start using py.test
from https://github.com/pydata/pandas/pull/13091

Currently _most_ of the tests pass with pytest, but there are 69 that don't. Most of these look like they're pickle or SQL / external resources issues.

pytest output here: https://gist.github.com/MaximilianR/225a071337e335ebdcc1d17637126126

So this does need a champion who could work through those issues. That's probably not me at the moment, but you have my moral support!

mpl is converting!
https://github.com/matplotlib/matplotlib/pull/5405
