Can't build with current Cython

[bensteinberg]

Our installations in local dev and CI started failing recently, I believe with the release of Cython 3.0.0 a couple of weeks ago: https://pypi.org/project/Cython/#history -- the symptom is a very long list of Cython compile errors. (I can attach an error log if desired.) I did the experiment of forking the repo and pinning Cython below 3, and builds started working again; I'll put in a PR accordingly.