Disable limited API for free-threaded Python

[rostan-t]

Free-threaded Python does not support the Limited C API.¹

This PR should make it possible to build OpenCV wheels for Python 3.13t (#1029).

Footnotes

1. https://docs.python.org/3/howto/free-threading-extensions.html#limited-c-api-and-stable-abi ↩

[rostan-t]

With this change, I was able to build a wheel for free-threaded Python with both pip wheel . and python setup.py bdist_wheel. It works with Python 3.13.2t, and I also tested it with older versions.

Is there any additional information I can provide to help move this forward?

[wk-xbkaishui]

hello, how to run the build process

[rostan-t]

hello, how to run the build process

This PR allows installing the library for free-threaded Python the same way as any version. This can be done like this:

pip install git+https://github.com/rostan-t/opencv-python.git@ft-python-limited-api

I can confirm that it works on x86_64 Linux but I haven't tried on other platforms.

[rostan-t]

Hi @sergregory, just checking in. Is there any more information I can provide to help move this forward? Thanks!

[rgommers]

This PR is correct, and merging it as is will prevent a build failure. By itself this PR doesn't make free-threaded Python supported but it's helpful to unblock other improvements, and cannot hurt. I recommend hitting the green button here.

[rgommers]

@asmorkalov sorry for the direct ping, but it would be really helpful if you could merge this one-liner fix - it avoids a build failure, and makes it possible to install directly from the default branch (or an sdist after the next release) in the absence of wheels for free-threaded Python.