Pin xarray to <2023.09.0

[pont-us]

Workaround for #897

Checklist:

• ~[ ] Add unit tests and/or doctests in docstrings~ n/a
• ~[ ] Add docstrings and API docs for any new/modified user-facing classes and functions~ n/a
• ~[ ] New/modified features documented in docs/source/*~ n/a
• ~[ ] Changes documented in CHANGES.md~ n/a
• [x] GitHub CI passes
• [x] AppVeyor CI passes
• [x] Test coverage remains or increases (target 100%)

[codecov[bot]]

Codecov Report

All modified lines are covered by tests :white_check_mark:

Comparison is base (fd1bbbe) 93.25% compared to head (9fbba79) 93.26%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #898 +/- ## ======================================= Coverage 93.25% 93.26% ======================================= Files 447 447 Lines 34712 34712 ======================================= + Hits 32372 32373 +1 + Misses 2340 2339 -1 ``` [see 1 file with indirect coverage changes](https://app.codecov.io/gh/dcs4cop/xcube/pull/898/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dcs4cop)

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