remove constraint to xarray with latest pypsa,linopy and xarray releases

[maurerle]

Description

We can unpin the xarray version, now that this is fixed upstream

Checklist

• Documentation updated (docstrings, READMEs, user guides, inline comments, doc folder updates etc.)
• New unit/integration tests added (if applicable)
• Changes noted in release notes (if any)
• Consent to release this PR's code under the GNU Affero General Public License v3.0

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 45.61%. Comparing base (095f342) to head (61f90d1).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #699   +/-   ##
=======================================
  Coverage   45.61%   45.61%           
=======================================
  Files          54       54           
  Lines        8030     8030           
=======================================
  Hits         3663     3663           
  Misses       4367     4367           

Flag	Coverage Δ
pytest	45.61% <ø> (ø)

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