Add 3.12 to version matrix for CI

[p-j-smith]

Ensure lipyphilic runs on 3.12

PR Checklist

• Tests added and passing?
• Docs added and building?
• CHANGELOG updated?
• AUTHORS updated if necessary?
• Issue raised and referenced?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.87%. Comparing base (3d9342e) to head (3a6156f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #131      +/-   ##
==========================================
- Coverage   98.88%   98.87%   -0.02%     
==========================================
  Files          19       19              
  Lines        1168     1155      -13     
  Branches      183      183              
==========================================
- Hits         1155     1142      -13     
  Misses         13       13              

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