Temporarily tolerate regression in xarray

[khaeru]

cf. #145

PR checklist

• Checks all ✅
• Update documentation N/A, test changes only
• Update doc/whatsnew.rst

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.80%. Comparing base (18d3ec2) to head (36043d5).
Report is 2 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##              main     #146      +/-   ##
===========================================
- Coverage   100.00%   99.80%   -0.20%     
===========================================
  Files           51       51              
  Lines         4695     4699       +4     
===========================================
- Hits          4695     4690       -5     
- Misses           0        9       +9     

Files with missing lines	Coverage Δ
genno/testing/__init__.py	100.00% <100.00%> (ø)
genno/tests/core/test_quantity.py	100.00% <100.00%> (ø)
genno/tests/test_operator.py	100.00% <100.00%> (ø)

... and 3 files with indirect coverage changes