html_url,issue_url,id,node_id,user,created_at,updated_at,author_association,body,reactions,performed_via_github_app,issue
https://github.com/pydata/xarray/pull/3526#issuecomment-554497403,https://api.github.com/repos/pydata/xarray/issues/3526,554497403,MDEyOklzc3VlQ29tbWVudDU1NDQ5NzQwMw==,3780274,2019-11-15T19:32:33Z,2019-11-15T19:32:33Z,NONE,"mmm, I am not sure I understand. Currently

```
ds = xr.Dataset()
ds.attrs['foo'] = {'bar': 1}
ds.to_zarr('foo.zarr', mode='w')
```

does not work.  This is because dict is not allowed in the list of types that the attributes are validated against. My feeling is this should not produce an error.

This PR just addresses that so the code above works. The following works as well:

```
ds = xr.open_zarr('foo.zarr')
ds.attrs['foo']
{'bar': 1}
```

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