DEPR: warnings from style deprecation only in PY3

[jreback]

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

[jreback]

this fixes: jreback@ad78ce2