Address typing hints for xarray 2023.08 compatibility, Update ReadTheDocs

[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)

• [x] 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?:

Fixes an issue with existing type hinting raised by changes in the API of xarray (2023.08).

Also updates the ReadTheDocs configuration to use Mambaforge by default.

• Does this PR introduce a breaking change?:

No. Only type hinting is affected (it's more accurate now).

• Other information:

https://github.com/pydata/xarray/pull/7368

[coveralls]

Pull Request Test Coverage Report for Build 5929676609

• 23 of 24 (95.83%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 78.485%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
clisops/core/subset.py	21	22	95.45%
<!--	Total:	23	24	95.83%	-->

Totals
Change from base Build 5718256149:	0.03%
Covered Lines:	1109
Relevant Lines:	1413

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💛 - Coveralls