Further fixes for sage-env-less installs #28320
Description
Follow-up for #28225
- Set SAGE_DOC as fallback for SAGE_DOC_SRC when sage-env is not available
- Fix sage-cython on python3
CC: @kiwifb @jhpalmieri
Component: distribution
Author: Antonio Rojas, François Bissey
Branch/Commit: 638a33b
Reviewer: John Palmieri
Issue created by migration from https://trac.sagemath.org/ticket/28320