[Cython] Preparing Cython 3.0 with breaking changes

[Stefan Behnel]

Hi all,

following the versioning discussion, I created a milestone to collect
(breaking) changes that should go into the future Cython 3.0 release.

https://github.com/cython/cython/milestone/58

While a major version change is a good time to fix things that, in
retrospect, have led us too far in the wrong direction, we are not planning
to remove the support for existing code any time soon that relies on the
current semantics. For now, it looks like the major breaking changes will
be to directive defaults, which means that setting them explicitly to their
current configuration (e.g. in setup.py) should keep code working across
Cython releases.

Feedback welcome.

Stefan