Support Cython 3.0+

[befeleme]

In Fedora Linux, we've successfully built httptools with Cython 3.0.5 (see build log for details).
Would you consider unpinning the requirement on Cython < 3 in the project? I can send the PR.

[tacaswell]

I can also confirm this.

Additionally, could you not include the generated c in the sdists? It causes problems when trying to install on newer versions of CPython.

[PaarthShah]

Yeah, I'm having trouble installing this on 3.13...

[DanielNoord]

Although not part of the project I think a PR would be welcome, as that is much easier to merge and review than doing that work yourself :)

[fantix]

Fixed in 0.6.2