expand test matrix to Python 3.10–3.12 and Ubuntu/macOS

[themattinthehatt]

Extends the test workflow from a single ubuntu-latest + Python 3.10 job to a 2×3 matrix
(ubuntu-latest/macos-latest × Python 3.10/3.11/3.12), giving coverage across supported Python versions
and confirming macOS compatibility. Codecov upload is restricted to the ubuntu-latest + 3.10 leg to avoid
duplicate/conflicting coverage reports.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.71%. Comparing base (44f4ad2) to head (fe64d19).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #100   +/-   ##
=======================================
  Coverage   90.71%   90.71%           
=======================================
  Files          16       16           
  Lines        1583     1583           
  Branches      179      179           
=======================================
  Hits         1436     1436           
  Misses        113      113           
  Partials       34       34           

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