Add xfail for issue 3209, and start testing Python 3.14

[musicinmybrain]

See commit messages and discussion in #3209 for details.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00000%. Comparing base (5eb5afb) to head (6b63492).
Report is 163 commits behind head on main.

Additional details and impacted files

@@               Coverage Diff               @@
##                 main        #3211   +/-   ##
===============================================
  Coverage   100.00000%   100.00000%           
===============================================
  Files             124          124           
  Lines           18787        18764   -23     
  Branches         1269         1269           
===============================================
- Hits            18787        18764   -23     

Files with missing lines	Coverage Δ
src/trio/_core/_tests/test_run.py	100.00000% <ø> (ø)

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[jakkdl]

codecov drop was due to the uploader giving an error 503 https://github.com/python-trio/trio/actions/runs/13333607547/job/37243603393?pr=3211#step:6:43
see #3173

[jakkdl]

I'm happily surprised none of the test_exports tests got upset!

[CoolCat467]

Looks good as far as I can tell

[trio-bot]

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