Pin xarray for xesmf-based regridding

[Zeitsperre]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes issue #xyz
• Tests for the changes have been added (for bug fixes / features)
• Documentation has been added / updated (for bug fixes / features)
• HISTORY.rst has been updated (with summary of main changes)
• I have added my relevant user information to AUTHORS.md

What kind of change does this PR introduce?:

• Emits a warning whenever attempting to use the regridding tools with newer xarray versions
• Fixes an import issue in ops
• Adds a pinned version of xarray to the extras recipe

Does this PR introduce a breaking change?:

No.

Other information:

[coveralls]

Pull Request Test Coverage Report for Build 7039086852

• 5 of 5 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall first build on open-xarray-pin at 72.112%

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Totals
Change from base Build 6981551428:	72.1%
Covered Lines:	1704
Relevant Lines:	2363

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💛 - Coveralls

[Zeitsperre]

@cehbrecht

This will tie-in with the changes on conda-forge to give us a workaround for now. The user should be made aware of these changes.