Add Python 3.14 free-threaded (cp314t) wheel support

## Summary

With the release of Python 3.14, the new free-threaded (no-GIL) mode is becoming more widely used. It would be great if pyroaring could provide prebuilt wheels and confirm compatibility with `python3.14t` (the free-threaded ABI).

## Current State

- PyPI shows wheels for cp313, cp312, cp311, but no cp314 or cp314t wheels
- Free-threaded Python uses the `cp314t` wheel tag (note the `t` suffix)

## What's Needed

1. Update CI (cibuildwheel) to build `cp314t` wheels
2. Declare GIL compatibility using `Py_mod_gil` slot in module definition (per PEP 703)
3. Test under free-threaded Python to ensure thread safety

## Resources

- [Python Free-Threading Guide](https://py-free-threading.github.io/)
- [Free-Threaded Wheels Tracker](https://hugovk.github.io/free-threaded-wheels/)
- [PEP 703 – Making the GIL Optional](https://peps.python.org/pep-0703/)
- [cibuildwheel free-threaded docs](https://cibuildwheel.pypa.io/en/stable/faq/#free-threaded-python)

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