Feat: free-threaded python support

[robsdedude]

This PR adds support for experimental no-GIL CPython builds (aka. free-threaded).

It gives special semantics to factors like py313t which will then only select free-threaded Python 3.13.

Note

This also means that factors like py313 will no longer match free-threaded CPython builds.

Related pypa/virtualenv#2809,

Closes: #3391

• ran the linter to address style issues (tox -e fix)
• wrote descriptive pull request text
• ensured there are test(s) validating the fix
• added news fragment in docs/changelog folder
• updated/extended the documentation

Please be aware that

• This is potentially a breaking change. If people relied on factors like py313 potentially also picking a free-threaded CPython build, that would break and they'd have to update their tox factor name to py313t. I'm not sure there's a design that circumvents that.
• This is the first time I'm digging through tox' code-base. So it might well be that I've missed places that need adjustment, too. In such case, I'm happy for any pointers.

[gaborbernat]

The approach looks good 😁

[robsdedude]

@gaborbernat that's interesting. The readthedocs build is failing because it's installing tox-uv which is very unhappy with this change as it's a breaking API change (PythonInfo gained a new field and constructor parameter: free_threaded). I never thought of tox as an API only CLI, but I suppose especially with plugins in mind, I better should have 😜

Now the question is how to best proceed. WDYT? Give the attribute a default value? Accept breaking the API? Something else entirely?

[gaborbernat]

Give the attribute a default value?

This 😊

[robsdedude]

I was about to pick this up again. Thanks for continuing. There was also another test I was about to adjust. Further, I wanted to discuss the fact that PythonInfo is a NamedTuple. So adding a field will break code that structurally unpacks it e.g., (impl, _, version, _, _, extra) = python_info. Up to you to decide how/whether to handle this API incompatibility. Lastly, I think the free_threaded info should also be included in the json log, but it's not yet (only in my local changes 😬 )

[gaborbernat]

I wanted to discuss the fact that PythonInfo is a NamedTuple. So adding a field will break code that structurally unpacks it e.g., (impl, _, version, _, _, extra) = python_info. Up to you to decide how/whether to handle this API incompatibility.

Yeah made it a dataclass. I'm fine breaking unpacking here.

There was also another test I was about to adjust.

Feel free to put in a new PR if there's anything else we can improve. Thanks!