Fix assignment with .loc

[dranjan]

• Closes Assigning with .loc indexing: implicit effect of dimension order? #7030
• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst

Apparently DataArray instances on the right-hand side of Variable.__setitem__ were being stripped of their xarray metadata, leading to the incorrect broadcasting noted in #7030. My proposed fix is to add an explicit if-clause for this case.

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[dranjan]

I realized that I hadn't paid much attention to Dataset vectorized assignment here. I took a look at the code and I think the fix implemented here should also work there, but it's probably worth adding a regression test for Dataset as well for the #7030 bug. That said, I don't want to hold up reviewing and merging this pull request, so I can do that in a separate small follow-up PR, or just put it here if that works. I would probably be getting to it later today after I leave work.

(By the way, I'm not sure what's happening with the Mypy CI checks, but I think it's not from this PR, because other recent PRs are also having the same failure.)

[dranjan]

I went ahead and added the test for Dataset. I figure putting it in this PR is less work for everyone.

[max-sixty]

I had a look through this, it looks good. Thanks @dranjan

I've been away for a while so want to be conservative about merging things myself — I'll see if anyone else comments, otherwise let's merge?

(and sorry for the delay)

[dranjan]

Awesome, I'm glad to see progress on this! Do you need anything more from me, like resolving that minor merge conflict?

[max-sixty]

I fixed the merge conflict and will merge. (If anyone more informed disagrees, we can revert in the worst case.)

Thank you very much @dranjan !

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