TST: suppress matplotlib warnings (or determine if they represent a bug)
There are a bunch of these in the test logs:

```
pandas/tests/plotting/test_hist_method.py::TestDataFramePlots::test_hist_column_order_unchanged[None-expected0]
pandas/tests/plotting/test_hist_method.py::TestDataFramePlots::test_hist_column_order_unchanged[column1-expected1]
pandas/tests/plotting/test_hist_method.py::TestDataFramePlots::test_hist_with_legend[None-None]
   pandas/tests/plotting/common.py:556: UserWarning: To output multiple subplots, the figure containing the passed axes is being cleared
    ret = f(**kwargs)
```

Do these represent problems we need to do something about?  cc @tacaswell 

If not, we should suppress them.  My attempts to do so with `pytestmark = pytest.mark.filterwarnings` have failed, could use another pair of eyeballs.