[Cython] Preparations for Cython 0.26.1 and 0.27

[Stefan Behnel]

Hi!

Given the current set of bugs and regressions in 0.26, I think we should
start preparing a 0.26.1 release. I've already updated the release branch
with recent fixes. It's currently the same as master, but I'd like to
separate the branches soon in order to start merging features for 0.27 to
avoid bit-rot.

The following regressions seem worth looking into for 0.26.1:

https://github.com/cython/cython/issues/1790

https://github.com/cython/cython/issues/1788

Anything else of importance?

Stefan