Add meson-python dep to unstable CI

[djhoese]

CI is failing in the unstable environment. I think it just needs this missing dependency (meson-python) for the mesonpy build backend.

• Closes #xxxx
• Tests added
• Tests passed
• Fully documented

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.68%. Comparing base (8aca19c) to head (e3d7f40).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #747   +/-   ##
=======================================
  Coverage   93.68%   93.68%           
=======================================
  Files          89       89           
  Lines       13721    13721           
=======================================
  Hits        12854    12854           
  Misses        867      867           

Flag	Coverage Δ
unittests	93.68% <ø> (ø)

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[djhoese]

Ok the unstable CI gets further but still fails as it is waiting on SciTools/cartopy#2690 to be released.