Hi FWIW, I recently did a test rebuild of packages in Ubuntu Mantic [1] (so packages are ~2.5 months behind unstable) against Cython 3.0.5.
Of the 279 source packages I identified as having a build-dependency on src:cython, 163 built successfully, 115 FTBFS and one (qutip) FTBFS on amd64 only and succeeded everywhere else. The package lists are too long to attach to a bug report, but I could attach them to an email to the debian-python mailing list, or so. I'd say this means we cannot easily switch to cython 3.0 right now. One option is to introduce a cython3.0 (or something better) package and maintainers can switch the build-dependency as needed. Cython3.0 might be confusing as the binary package from src:cython is already named cython3. Regards Graham [1] https://launchpad.net/~ginggs/+archive/ubuntu/cython-3.0
