Fix CI shell handling for nox tests

[mcflugen]

I've fixed the CI test job so that nox now uses the Python version selected by actions/setup-python rather than the system Python. I also dropped Python 3.11.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (c4d8acc) to head (2cd9e0a).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #43   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines          460       460           
=========================================
  Hits           460       460           

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