(Closes #494) Fix Black checks in CI and update to use 3.14

[arporter]

No description provided.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.19%. Comparing base (58eae20) to head (384d2a5).
⚠️ Report is 7 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #495   +/-   ##
=======================================
  Coverage   92.19%   92.19%           
=======================================
  Files          86       86           
  Lines       13807    13807           
=======================================
  Hits        12730    12730           
  Misses       1077     1077           

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[arporter]

A 'small' PR in the sense that it really just updates the GitHub workflow to ensure that we specify which version of Python we're using for the Black test. It also updates the CI to use 3.14 in place of 3.13 in the unit tests.
I've updated the dev. guide a bit as the fact that Black's formatting depends on Python version could catch developers out.
Ready for a first look from @sergisiso @LonelyCat124 or @hiker :-)

[sergisiso]

The CI is now green again. Thanks for also updating the documentation. This is approved for merging @arporter