Does not build with cython 3.1.x

[bbhtt]

        * Getting build dependencies for wheel...
        ERROR Missing dependencies:
        	cython<3.1.0,>=3.0.2
        Command 'python -P -mbuild --no-isolation --wheel --outdir /buildstream-build/dist .' failed with exitcode 1

Any chance the constraints can be relaxed?

I built it with 3.1.2 and it seems to build ok.