DEPR: warnings from style deprecation only in PY3
https://travis-ci.org/pydata/pandas/jobs/108709963

```
S.......S.S........................................................................./home/travis/build/pydata/pandas/pandas/core/config.py:219: FutureWarning: 
mpl_style had been deprecated and will be removed in a future version.
Use `matplotlib.pyplot.style.use` instead.
  return self.__func__(*args, **kwds)
```

cc @TomAugspurger 

prob should bump the stacklevel as well
