Test as well on the free-threaded version of Python 3.14

[tleonhardt]

Python 3.14 will officially support free-threading, removing the "experimental" tag from the free-threaded build. This means it's no longer experimental, but it won't be the default build. The transition to free-threading in Python involves multiple phases, and 3.14 marks the beginning of phase II, where free-threading is officially supported but remains optional.

This PR simply adds 3.14t, the free-threaded version of Python 3.14 as an additional test target Python version.

It also updates various files to note that we explicitly support free-threaded versions of Python.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.57%. Comparing base (eb5eb6c) to head (99039a4).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1450      +/-   ##
==========================================
+ Coverage   98.55%   98.57%   +0.01%     
==========================================
  Files          20       20              
  Lines        5742     5742              
==========================================
+ Hits         5659     5660       +1     
+ Misses         83       82       -1     

Flag	Coverage Δ
unittests	98.57% <ø> (+0.01%)	⬆️

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