Remove Panel reference

[fl4p-old]

.../python3.7/site-packages/arctic/store/_pandas_ndarray_store.py:6: FutureWarning: The Panel class is removed from pandas. Accessing it from the top-level namespace will also be removed in the next version
  from pandas import DataFrame, Series, Panel

Used by PandasPanelStore https://github.com/manahl/arctic/blob/master/arctic/store/_pandas_ndarray_store.py#L209

[upwell]

arctic/_util.py:6: FutureWarning: pandas.util.testing is deprecated. Use the functions in the public API at pandas.testing instead.
  from pandas.util.testing import assert_frame_equal

Here is another future warning when import arctic.

[enricodetoma]

Please have a look at my pull request which should fix these import errors/warnings