Add support for Python 3.14, upgrade dependencies and CI actions, upgrade macOS support to 14.0+, add free-threaded CPython

[benoit74]

Fix #236
Fix #235

Changes:

• Drop support of Python 3.9
• Add support for Python 3.14
• Upgrade dependencies, especially Cython 3.1.6
• Upgrade supported macOS to 14.0+ (instead of 13.0+)
• Add support for free-threaded CPython (3.13 and 3.14)
• Upgrade Github CI Actions
• Run tests on minimum supported platforms + more recent stable ones

Notes:

• I had to drop support of Python 3.9 because (at least) coverage 7.11.0 supports only Python >= 3.10 (and 3.9 is anyway EOL)
• Upgrade to Cython 3.1+ needed special attention for Python 3.13+ ; if we agree on the approach, I will open an issue to not forget to "rollback" this once sys.monitoring works correctly with coverage and Cython
• I'm also going to open an issue regarding GIL and free-threaded Python ; work on this PR is conservative to continue same approach done so far, but I'm questioning this is the optimal approach

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.69%. Comparing base (1004cf3) to head (82b5eac).
⚠️ Report is 9 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #237      +/-   ##
==========================================
+ Coverage   93.66%   93.69%   +0.02%     
==========================================
  Files           1        1              
  Lines         537      539       +2     
==========================================
+ Hits          503      505       +2     
  Misses         34       34              

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[rgaudin]

Great!