Unpin xarray
- [X] closes [#42716](https://github.com/pandas-dev/pandas/issues/42716) 
- [ ] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [X] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [ ] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

> looks like this is breaking a doctest example

xref https://github.com/pandas-dev/pandas/issues/42716#issuecomment-886271371 for the doctest issue (the mypy part seems fine now) It seems that the CircleCI: test-arm is not passing. Upon inspecting the error, it seems there might be a [numpy dependency missing](https://app.circleci.com/pipelines/github/pandas-dev/pandas/11048/workflows/10dc60d8-ff99-407f-8f7d-ee5d065df965/jobs/41887?invite=true#step-102-1166). I think that the previous doctest issue is fixed now, but will check back in the morning.

