Not exactly what you’re asking for, but one option would be to constrain the version of Cython it requires, e.g. Cython >= 0.29, <1, and extend it when Cython 3.0 is out and you can check that it works. This also protects it against whatever changes in Cython 4.0.
Of course, there’s always a downside: your package might fail to build on a new version of Python if a new version of Cython is needed to do so. Capping the supported Python version is not a good idea because of the way the resolver responds to that - see this thread.