id,node_id,number,title,user,state,locked,assignee,milestone,comments,created_at,updated_at,closed_at,author_association,active_lock_reason,draft,pull_request,body,reactions,performed_via_github_app,state_reason,repo,type
502720385,MDU6SXNzdWU1MDI3MjAzODU=,3374,Key error in to_netcdf ,6943441,closed,0,,,5,2019-10-04T16:04:30Z,2023-03-14T21:14:33Z,2023-03-14T21:14:33Z,NONE,,,,"#### MCVE Code Sample
<!-- In order for the maintainers to efficiently understand and prioritize issues, we ask you post a ""Minimal, Complete and Verifiable Example"" (MCVE): http://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports -->

```python
import xarray as xr

p = range(10)
ds = xr.Dataset({'precip': (['time'], p)}, coords={'time': range(10)})
ds.attrs['Conventions'] = ['COARDS', 'GrADS']
ds.to_netcdf('a.nc')
```

#### Expected Output
Saves data to `a.nc` without error

#### Problem Description
<!-- this should explain why the current behavior is a problem and why the expected output is a better solution -->
In version `0.12.1` this works without error. In `0.12.2` it throws an error:

```
  File ""xarray_error.py"", line 6, in <module>
    ds.to_netcdf('a.nc')
  File ""C:\Users\jsadler\AppData\Local\Continuum\anaconda3\envs\nwm\lib\site-packages\xarray\core\dataset.py"", line 1365, in to_netcdf
    compute=compute)
  File ""C:\Users\jsadler\AppData\Local\Continuum\anaconda3\envs\nwm\lib\site-packages\xarray\backends\api.py"", line 900, in to_netcdf
    store.close()
  File ""C:\Users\jsadler\AppData\Local\Continuum\anaconda3\envs\nwm\lib\site-packages\xarray\backends\scipy_.py"", line 226, in close
    self._manager.close()
  File ""C:\Users\jsadler\AppData\Local\Continuum\anaconda3\envs\nwm\lib\site-packages\xarray\backends\file_manager.py"", line 188, in close
    file.close()
  File ""C:\Users\jsadler\AppData\Local\Continuum\anaconda3\envs\nwm\lib\site-packages\scipy\io\netcdf.py"", line 299, in close
    self.flush()
  File ""C:\Users\jsadler\AppData\Local\Continuum\anaconda3\envs\nwm\lib\site-packages\scipy\io\netcdf.py"", line 409, in flush
    self._write()
  File ""C:\Users\jsadler\AppData\Local\Continuum\anaconda3\envs\nwm\lib\site-packages\scipy\io\netcdf.py"", line 420, in _write
    self._write_gatt_array()
  File ""C:\Users\jsadler\AppData\Local\Continuum\anaconda3\envs\nwm\lib\site-packages\scipy\io\netcdf.py"", line 442, in _write_gatt_array
    self._write_att_array(self._attributes)
  File ""C:\Users\jsadler\AppData\Local\Continuum\anaconda3\envs\nwm\lib\site-packages\scipy\io\netcdf.py"", line 450, in _write_att_array
    self._write_att_values(values)
  File ""C:\Users\jsadler\AppData\Local\Continuum\anaconda3\envs\nwm\lib\site-packages\scipy\io\netcdf.py"", line 562, in _write_att_values
    nc_type = REVERSE[values.dtype.char, values.dtype.itemsize]
KeyError: ('U', 24)
```

If instead the attribute is saved as as string instead of a list, it works:
```python
import xarray as xr

p = range(10)
ds = xr.Dataset({'precip': (['time'], p)}, coords={'time': range(10)})
ds.attrs['Conventions'] = ""['COARDS', 'GrADS']""
ds.to_netcdf('a.nc')
```

#### Output of ``xr.show_versions()``
<details>
INSTALLED VERSIONS
------------------
commit: None
python: 3.7.4 (default, Aug  9 2019, 18:34:13) [MSC v.1915 64 bit (AMD64)]
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 158 Stepping 10, GenuineIntel
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: None.None
libhdf5: None
libnetcdf: None

xarray: 0.12.2
pandas: 0.25.1
numpy: 1.17.1
scipy: 1.3.1
netCDF4: None
pydap: installed
h5netcdf: None
h5py: None
Nio: None
zarr: 2.3.2
cftime: None
nc_time_axis: None
PseudonetCDF: None
rasterio: None
cfgrib: None
iris: None
bottleneck: None
dask: 2.3.0
distributed: 2.5.1
matplotlib: 3.1.1
cartopy: None
seaborn: None
numbagg: None
setuptools: 41.2.0
pip: 19.2.3
conda: None
pytest: 5.1.2
IPython: 7.8.0
sphinx: None

</details>
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