CI: enable free-threaded wheel builds [wheel build]

[ngoldbaum]

I think all that's needed is this one-line change and the 3.13 wheel builder will automatically create a free-threaded wheel. Let's see...

[ngoldbaum]

Unfortunately I think we might need a cython release that builds numpy under free-threaded python. At least if we want to avoid hacks like the change I made to pyproject.toml, which I definitely don't think is a good idea.

I don't think there's any other way to customize the build requirements? Could I maybe do a sed replacement in pyproject.toml on the build image in the cibw_before_build.sh script?

[ngoldbaum]

Looks like the wheels successfully built for manylinux and musllinux: https://github.com/numpy/numpy/actions/runs/9215227199

[rgommers]

This is indeed not mergeable with the change to pyproject.toml. In addition, we may not want to have the cp313t builds in wheel.yml, because I don't think we want to upload them to PyPI for 2.1.0. So how about this:

• copy wheels.yml with these changes to free_threaded.yml
• edit that file so it only contains what is needed for free-threaded builds (no sdist, no other platforms, no upload to the STAGING bucket)
• set the PIP_NO_BUILD_ISOLATION=1 env var
• then build Cython from latest master (no change to pyproject.toml needed anymore, because of turning off build isolation)

Then we can merge this and start uploading to https://anaconda.org/scientific-python-nightly-wheels/numpy

[ngoldbaum]

Then we can merge this and start uploading to https://anaconda.org/scientific-python-nightly-wheels/numpy

I ended up doing this in a slightly different way because setting PIP_NO_BUILD_ISOLATION messes with cibuildwheel's assumptions for the build environment, but it works out to the same thing more or less.

The tests pass on both manylinux and musllinux with the GIL disabled - I think the wheels should be ready to upload along with the nightlies.

[rgommers]

LGTM! All seems to work as advertised, nice work Nathan. Please feel free to merge once the comment cleanup is done.

[ngoldbaum]

Note for anyone curious: I'm leaving the windows and mac-specific stuff in here even though it's unused because our intention is to add mac builds and then windows builds when that's possible.