As suggested in #30371, the cython config is extracted to a new module to make it easier to reuse it in the editable install.

CC: @mkoeppe

Component: build

Author: Tobias Diez

Branch/Commit: cc96c6d

Reviewer: Matthias Koeppe

Issue created by migration from https://trac.sagemath.org/ticket/30709