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
936123457,MDExOlB1bGxSZXF1ZXN0NjgyOTI5NDA1,5569,Consolidate TypeVars in a single place,5635139,closed,0,,,9,2021-07-03T00:01:09Z,2021-08-22T00:04:54Z,2021-08-21T23:38:14Z,MEMBER,,0,pydata/xarray/pulls/5569,"<!-- Feel free to remove check-list items aren't relevant to your change -->

- [x] Passes `pre-commit run --all-files`

This consolidates the TypeVars into a single place, and merges `VariableType` and `T_Variable`.

I'm not 100% sure the interaction between the `.pyi` file and the new `xarray.core.types` is optimal. But it does seem to work. Feedback very welcome, as ever.

There's a note to rename `T_DSorDA` — what do people think? `T_Data`? It's kinda inscrutable atm.
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