Remove remaining usage of pandas Panel

[jorisvandenbossche]

There is still some usage of pd.Panel (just accessing the name in the pandas namespace for isinstance check), eg:

xarray/xarray/core/merge.py

Line 18 in a78e0f6

PANDAS_TYPES = (pd.Series, pd.DataFrame, pd.Panel)

We are currently removing Panel from pandas (pandas-dev/pandas#27101), so that breaks xarray (and statsmodels). For that reason, we are going to keep an empty class so that the above use case doesn't directly break. But it would still be good to remove that usage (or only add it for pandas versions below 0.25.0)

[shoyer]

Thanks for the heads up! This will be fixed in #3058