ci: Add Python 3.13t and 3.14t (free-threaded) to testing matrix

[xsub]

This PR adds Python 3.13t and 3.14t (the experimental free-threaded builds) to the testing matrix.

Since Python 3.13, PEP 703 introduces a GIL-less build. Adding this to the CI early helps uncover hidden data-races and ensures thread-safety without the GIL. The actions/setup-python action supports 3.13t out of the box.

Note: asyncssh relies on cryptography, which currently lacks full free-threaded ABI support (see pyca/cryptography#15063). This CI run will track upstream compatibility and serve as a baseline diagnostic.

If the tests pass without the GIL, the project is safely compatible with the free-threaded ecosystem. If they fail, this run provides high-value logs regarding global mutable state or dependency blockers.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.84%. Comparing base (fab96ac) to head (07b38ca).
⚠️ Report is 197 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #817      +/-   ##
===========================================
- Coverage    99.85%   99.84%   -0.02%     
===========================================
  Files           95      102       +7     
  Lines        27784    32034    +4250     
  Branches      2908     3096     +188     
===========================================
+ Hits         27743    31983    +4240     
- Misses          38       45       +7     
- Partials         3        6       +3     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[ronf]

Looks like all of the new tests failed, but on Ubuntu and Windows they failed very early, when trying to find the right Python environment. It looks like macOS got a bit further and began running the tests, but there were crashes.