dataset attrs list of strings to_netcdf() error

What happened?

when I try and save the dataset with an attribute as a list of strings it fails.

See example code

import xarray as xr
import pandas as pd
import numpy as np

ds = xr.Dataset()

temp = 15 + 8 * np.random.randn(2, 2, 3)

precip = 10 * np.random.rand(2, 2, 3)

lon = [[-99.83, -99.32], [-99.79, -99.23]]

lat = [[42.25, 42.21], [42.63, 42.59]]

ds["temperature"] = (("x", "y", "time"), temp)

ds["temperature_double"] = (("x", "y", "time"), temp * 2)

ds["precipitation"] = (("x", "y", "time"), precip)

ds.coords["lat"] = (("x", "y"), lat)

ds.coords["lon"] = (("x", "y"), lon)

ds.coords["time"] = pd.date_range("2014-09-06", periods=3)

ds.coords["reference_time"] = pd.Timestamp("2014-09-05")

ds.attrs['test'] = [1,2,3]
ds.to_netcdf('test.nc')
# no error

ds.attrs['test'] = ['1','2','3']
ds.to_netcdf('test.nc')

  File "C:\Users\xxxx\Anaconda3\envs\base1\lib\site-packages\scipy\io\_netcdf.py", line 555, in _write_att_values
   nc_type = REVERSE[values.dtype.char, values.dtype.itemsize]

 KeyError: ('U', 4)

What did you expect to happen?

No response

Minimal Complete Verifiable Example

No response

MVCE confirmation

• Minimal example — the example is as focused as reasonably possible to demonstrate the underlying issue in xarray.
• Complete example — the example is self-contained, including all data and the text of any traceback.
• Verifiable example — the example copy & pastes into an IPython prompt or Binder notebook, returning the result.
• New issue — a search of GitHub Issues suggests this is not a duplicate.

Relevant log output

No response

Anything else we need to know?

No response

Environment

Start at Dataset.to_netcdf() and the scipy/io/_netcdf.py _write_att_values traceback shown in the report. Reproduce the two ds.attrs['test'] assignments and compare the successful numeric-list write with the failing string-list write. Done means the string-list attribute case writes successfully, with a regression test covering it.