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     

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[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.

[xsub]

Hi @ronf, thanks for looking into this so quickly!

The behaviors you're seeing match the current state of the broader PEP 703 migration ecosystem:

1. Ubuntu/Windows Early Failures: This is an upstream packaging and infrastructure issue. Because libraries like cryptography don't yet publish pre-built wheels (.whl) for 3.13t and 3.14t, the CI attempts to compile them from source. This frequently fails immediately on Linux and Windows runners due to incomplete free-threaded CFFI or PyO3 build support.
2. macOS Crashes: The macOS runner managed to compile the dependencies and actually execute the test suite. The crashes you're seeing during the tests are likely true free-threading concurrency faults (e.g., Segmentation Faults). Without the GIL, the underlying C-extensions temporarily lose their implicit thread-safety, causing memory access violations during async operations.

Since we know this is currently blocked at the upstream extension level (specifically abi3t support tracked in pyca/cryptography#15063), I suggest we convert this PR to a Draft.

We can leave it open as an automated tracker. Once cryptography and other core dependencies release their free-threaded wheels, we can re-trigger this workflow to verify if asyncssh becomes natively GIL-free or if further patches are needed.

What do you think about that?

[ronf]

That sounds good to me... I'm fine with keeping this open in the meantime. I've gone ahead and made this into a Draft.

Thanks for submitting this. I have seen comments about the free-threading work, but haven't tracked it closely.