Make Cython a build requirement

[IAlibay]

We've been saying we'd do this for ages but never go around to it.

Making Cython required at build time (which is sorta is because of pyproject.toml) would make deployment life a lot easier.

Are there any downsides here? Do we maybe need to pin to a very specific Cython version on release or can we assume it should be fine as long as it's more or less in the right range?

[richardjgowers]

I think we've always been waiting for v3.0 (i.e. semver stable) to be released. That said, I don't think a) we're using cython too "aggressively" such that it would break version to version and b) cython has never given us troubles wrt versions. It's probably fine just to have cython as a build requirement

[hmacdope]

@IAlibay shall I add this to the 3.0.0 milestone? Remove if you think it should be push back.

[IAlibay]

Mayybe? I might revisit it for 2.5.0 tbh, it's only really affecting tarball installs at this rate I think.

[IAlibay]

I think we've just been overlooking that Cython has been a build requirement since we've switched to having a pyproject.toml file. I'm going to put this as a 2.5.0 target, we should solve this before our setup.py ends up causing us actual trouble.

[IAlibay]

Moving this to 2.6.0, there's a lot of setup.py stuff that needs untangling that won't be resolved yet.

[hmacdope]

Adding to 3.0 board.