Add extra OS to test strategy

[znicholls]

Description

Checklist

Please confirm that this pull request has done the following:

• Tests added N/A
• Documentation added (where applicable) N/A
• Changelog item added to changelog/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.98%. Comparing base (1d3a0ae) to head (fbc68f3).
⚠️ Report is 136 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #21   +/-   ##
=======================================
  Coverage   96.98%   96.98%           
=======================================
  Files          27       27           
  Lines        1358     1358           
  Branches      153      153           
=======================================
  Hits         1317     1317           
  Misses         23       23           
  Partials       18       18           

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[znicholls]

No idea why numpy was causing the failure, but it seems to be, see these two runs:

• https://github.com/openscm/pandas-openscm/actions/runs/16314507230/job/46077068990?pr=21
• https://github.com/openscm/pandas-openscm/actions/runs/16314832204/job/46078132628?pr=21

Will now try and release and see how we go with PyPI install

[znicholls]

GitHub spinning, has been merged