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
1251511305,PR_kwDOAMm_X844nz8j,6651,Typing support for custom backends,43316012,closed,0,,,4,2022-05-28T07:28:41Z,2022-06-04T10:17:55Z,2022-05-28T10:29:16Z,COLLABORATOR,,0,pydata/xarray/pulls/6651,"<!-- Feel free to remove check-list items aren't relevant to your change -->

- [x] Closes #6632

So far we have not found a good way of typing dynamically added custom backends (installed via pip).
So the only fallback option is to allow str.

I have decided to leave the Literal as type, since it gives the user a nice list of supported backends in the editor.
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