Add support for Cython==3.x.x and increase cython_min version

[misl6]

Maintainer merge checklist

• Title is descriptive/clear for inclusion in release notes.
• Applied a Component: xxx label.
• Applied the api-deprecation or api-break label.
• Applied the release-highlight label to be highlighted in release notes.
• Added to the milestone version it was merged into.
• Unittests are included in PR.
• Properly documented, including versionadded, versionchanged as needed.

🟢 Everything has been tested on Cython==3.0.0 and Cython==0.29.36 on Apple Silicon (mac)
⚠️ Let's see if there's something else to migrate for other platforms via CI

• self.sources = args[1] raises an error with Cython==3.0.0 as sources is a kwarg instead, but that does not seem needed anymore.

• cpdef can't be used for variables (and was not needed)

• Cython==3.0.0 is less permissive with implicit casting

• Increased cython_max to 3.0.0 and cython_min to 0.29.1 (previous Cython versions are not compatible with any of the currently supported Python versions)