xr.DataSet.from_dataframe / xr.DataArray.from_series does not preserve DateTimeIndex with timezone

Problem Description

When using DataSet.from_dataframe (DataArray.from_series) to convert a pandas dataframe with DateTimeIndex having a timezone - xarray convert the datetime into a nanosecond index - rather than keeping it as a datetime-index type.

MCVE Code Sample

print( df.index ) 

DatetimeIndex(['2000-01-03 16:00:00-05:00', '2000-01-03 16:00:00-05:00',
               '2000-01-03 16:00:00-05:00', '2000-01-03 16:00:00-05:00',
               ...
               '2019-08-20 16:00:00-05:00', '2019-08-20 16:00:00-05:00'],
              dtype='datetime64[ns, EST]', name='time', length=12713014, freq=None)

ds = xr.DataSet.from_dataframe( df.head( 1000 )  ) 
print( ds['time'] )

<xarray.DataArray 'time' (time: 7)>
array([946933200000000000, 947019600000000000, 947106000000000000,
       947192400000000000, 947278800000000000, 947538000000000000,
       947624400000000000, ...], dtype=object)
Coordinates:
  * time     (time) object 946933200000000000 ... 947624400000000000

Expected Output

After removing the tz localization from the DateTimeIndex of the dataframe , the conversion to a DataSet preserves the time-index (without converting it to nanoseconds)

df.index = df.index.tz_convert('UTC').tz_localize(None)
ds = xr.DataSet.from_dataframe( df.head(1000) ) 
print( ds['time] )

<xarray.DataArray 'time' (time: 7)>
array(['2000-01-03T21:00:00.000000000', '2000-01-04T21:00:00.000000000',
       '2000-01-05T21:00:00.000000000', '2000-01-06T21:00:00.000000000',
       '2000-01-07T21:00:00.000000000', '2000-01-10T21:00:00.000000000',
       '2000-01-11T21:00:00.000000000'], dtype='datetime64[ns]')
Coordinates:
  * time     (time) datetime64[ns] 2000-01-03T21:00:00 ... 2000-01-11T21:00:00

Output of xr.show_versions()

Start by running the MCVE against DataSet.from_dataframe and DataArray.from_series with the timezone-aware pandas DatetimeIndex shown in the issue. Trace those conversion entry points and verify that the resulting time coordinate preserves datetime values and timezone information rather than becoming nanosecond integers; compare it with the timezone-localized example as the expected behavior.