Build wheels for Python 3.13

[iisakkirotko]

Hi! With python 3.13 coming out (hopefully) later today, it would be nice to have wheels for the new version available.

This PR adds Python 3.13 to the versions for which tiktoken should be built (for now together with CIBW_PRERELEASE_PYTHONS: True, since only 3.13.0rc2 is available to date). It also bumps pyo3 to version 0.22 which supports Python 3.13, together with the gil-refs feature flag for backwards compatibility.

I suppose a discussion could be had about whether support for Python 3.8 should be dropped due to its end-of-life, but that's for another day.

Let me know if there's anything else that should be accounted for, my experience with Rust is very limited :)

[hauntsaninja]

Thanks for the PR! The internal version already uses the new PyO3 Bound API, I'll sync the codebase.
Not sure what's regressed the QEMU aarch64 builds

[hauntsaninja]

tiktoken 0.8 ships 3.13 wheels. Needed a few things to be different from this diff. Thanks again!