Remove explicit references to Pandas in dask.dataframe.groupby (#4778)
Remove explicit references to Pandas in dask.dataframe.groupby (#4778)

* Remove explicit references to Pandas in dask.dataframe.groupby

* Add comment explaining behavior [skip ci]

blaze/dask,dask/dask,ContinuumIO/dask,dask/dask,blaze/dask,mrocklin/dask,ContinuumIO/dask,jakirkham/dask,jakirkham/dask,mrocklin/dask