feat: add Python 3.15 support

[HugoMVale]

• Add 3.15 classifier and CI test matrix entry (lowest-direct excluded, as for 3.12-3.14)
• Bump cibuildwheel to v4.3.1, which builds CPython 3.15 wheels by default

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.59%. Comparing base (c3267d0) to head (6466e51).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #16   +/-   ##
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  Coverage   99.59%   99.59%           
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  Files           4        4           
  Lines         244      244           
  Branches       63       63           
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  Hits          243      243           
  Partials        1        1