Xarray indexing compatibility

[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)

• [x] HISTORY.rst has been updated (with summary of main changes)

• [x] I have added my relevant user information to AUTHORS.md

• What kind of change does this PR introduce?:

• Backwards-compatible indexing changes to support newest xarray in core.subset.

• Adds the packaging library for consistent version parsing

• Fixes a small grammar mistake in a docstring

• Does this PR introduce a breaking change?:

Yes. Dependencies have been affected.

• Other information:

https://github.com/pydata/xarray/commit/3ead17ea9e99283e2511b65b9d864d1c7b10b3c4

[Zeitsperre]

@cehbrecht Would it be possible to pin a release after merging this PR? This has impacts on our PAVICS stack.

CC: @tlvu