After #33011, downstream packages that use Cython and cimport Sage modules need to activate PEP 420 namespace package support in Cython using with cython_namespace_package_support (as is done in https://github.com/sagemath/sage-prod/blob/develop/src/setup.py#L106 for sagelib).

This feature is standard in the upcoming Cython 3 (#29863). Until downstream packages are updated, we can patch our Cython.

Upstream backport PR:

• Backport to 0.29.x: Add PEP420 namespace support cython/cython#4918

Depends on #34237

CC: @culler @tscrim @videlec @jhpalmieri

Component: packages: standard

Author: Matthias Koeppe

Branch/Commit: 48a9ba2

Reviewer: Marc Culler, John Palmieri

Issue created by migration from https://trac.sagemath.org/ticket/34221