Add Python 3.12 to CI testing

[djhoese]

• Closes #xxxx (remove if there is no corresponding issue, which should only be the case for minor changes)
• Tests added (for all bug fixes or enhancements)
• Tests passed (for all non-documentation changes)
• Passes git diff origin/master **/*py | flake8 --diff (remove if you did not edit any Python files)
• Fully documented (remove if this change should not be visible to users, e.g., if it is an internal clean-up, or if this is part of a larger project that will be documented later)

[djhoese]

Closing/reopening to restart CI

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (6617ff7) 91.23% compared to head (c7b179a) 91.48%.
Report is 35 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #148      +/-   ##
==========================================
+ Coverage   91.23%   91.48%   +0.24%     
==========================================
  Files          11       12       +1     
  Lines        3867     3956      +89     
==========================================
+ Hits         3528     3619      +91     
+ Misses        339      337       -2     

Flag	Coverage Δ
unittests	91.48% <ø> (+0.24%)	⬆️

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[coveralls]

coverage: 91.713%. remained the same
when pulling c7b179a on djhoese:ci-py312
into 69793ca on pytroll:main.