Include Python 3.14 into unit test matrix

[soininen]

Re spine-tools/Spine-Toolbox#3189

Checklist before merging

• Unit tests pass

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.66%. Comparing base (e70a756) to head (50c4e12).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #606   +/-   ##
=======================================
  Coverage   85.66%   85.66%           
=======================================
  Files          81       81           
  Lines       11040    11040           
  Branches     1623     1623           
=======================================
  Hits         9457     9457           
  Misses       1227     1227           
  Partials      356      356           

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