Support building free-threaded CPython

[colesbury]

Add support for Python's free threading build mode where the global interpreter lock is disabled. The packages are marked using a suffix on the architecture, like 'x64-freethreaded' or 'arm64-freethreaded'.

See actions/setup-python#771.

[colesbury]

@mayeut suggested using the "arch" field in actions/setup-python#771 (comment). It's not a perfect fit, but it seems better than any other option to me.

In particular, I experimented with using "+freethreaded" as a semver build metadata like uv does (e.g., "3.13.0+freethreaded"), but that doesn't work because:

1. Other parts of the GitHub actions toolchain strip build variants by calling semver.clean()
2. Semver specifies that build metadata must be ignored when determining version precedence, which makes it an awkward fit

[mayeut]

Regarding the macOS & Windows installs, I think it uninstalls before installing which might mess up things depending on how setup-python is called (especially with multiple installs at once or in multiple steps).

For Windows:
I'm almost sure one easy fix would be to use the official nugget package as a source: https://www.nuget.org/packages/python-freethreaded/3.13.0

This would remove all the uninstall & registry bits (this last one might interfere with the py launcher though but given this adds support for a new variant, I don't think it matters much for now).

cibuildwheel uses this source for Windows CPython: https://github.com/pypa/cibuildwheel/blob/5cdddb2d4696110e00029b0b615799855912564d/cibuildwheel/windows.py#L108-L121

Building the package could be installing the nugget package to a temp dir & zipping the correct folder.
It would require a new install script that just creates symlinks, install/update pip as is done on Linux.

[colesbury]

@mayeut - I'm not sure I understand your concern. The uninstall step is part of creating the python-versions package, not setup-python. I don't see the problem and using a different installation method just for the freethreading package seems more risky and harder to maintain.

You can run setup-python with multiple versions, including 3.13t. See:

• https://github.com/colesbury/setup-python-test/actions/runs/11820601328
• https://github.com/colesbury/setup-python-test/actions/runs/11820589990

[mayeut]

@colesbury, you're right. I misread the setup.ps1, fiddling with the registry keys in fact prevents uninstall (but still messes up registry keys).

That being said, there's another point worth mentioning, the free threaded binaries are not embedded in the package but they are downloaded when installing (the offline installer only has the default options embedded, that means no free threaded binaries). Using the nuget installation method, everything needed will be in the built package without the need to download anything else upon install by setup-python.
Reading https://docs.python.org/3/using/windows.html, nuget packages are meant for CI and allows side-by-side installations out-of-the box (which is exactly the use case of setup-python).
As everything is working right now, probably not worth getting this in while waiting for maintainers feedback.

Running setup-python with multiple versions fails on Linux: https://github.com/mayeut/sandbox/actions/runs/11825252827/job/32948605164
colesbury#9