Consider switching all geodataframe tests to use gpd instead of pd asserts

[krivard]

#617 switches from pd.testing.assert_frame_equal to gpd.testing.assert_geodataframe_equal when comparing data frames with polygons and multipolygons in test__join_county_geo_df. This was necessary due to a (probable) change in representation that the pandas assert couldn't interpret properly.

For consistency, we may want to change the remaining applicable pandas asserts in Python-packages/covidcast-py/tests/test_plotting.py to geopandas asserts.