Don't use SAGE_CYTHONIZED in sage_include_directories(sources=True) #23744
Description
Currently, SAGE_CYTHONIZED is used by src/sage/env.py to determine some include directories while building Sage.
Since we want to get rid of SAGE_CYTHONIZED (#21535), a first step would be to move this bit to setup.py in order to concentrate all uses of SAGE_CYTHONIZED in one place.
Component: build
Author: Jeroen Demeyer
Branch/Commit: 33746a2
Reviewer: Erik Bray
Issue created by migration from https://trac.sagemath.org/ticket/23744