[Cython] Cython 0.29.5 fixes a crash for cpdef functions

[Stefan Behnel]

Hi everyone,

Cython 0.29.5 was released with an important bug fix for extension types
that have "cpdef" methods. If they get subclassed by Python classes that do
not define additional attributes (or anything that would require them to
have an instance `__dict__`), then calling the cpdef-methods repeatedly
triggers a crash in the optimised code in 0.29.x that checks for subclass
overrides. The issue was reported here:

https://github.com/cython/cython/issues/2823

It's a new issue in the 0.29.x series and only appears under Python 3.6 and
later, so older versions were not affected. Everyone using 0.29.x with
cpdef methods is encouraged to upgrade.

Stefan