pkgs/sagemath-*/tox.ini: Create environment that uses Sage's python as the basepython, fix sagemath-standard deps #32716
Description
Newer versions of tox seem to provision python 3.10 when the environment python is used, but then our wheels don't necessarily fit.
For this reason, the examples shown in the developer's manual hardcoded the Python version (tox -e py39).
We now define tox environments sagepython in tox.ini, which takes Python as SAGE_VENV/bin/python3 and change the manual to use this environment in examples, such as:
./bootstrap && ./sage -sh -c '(cd pkgs/sagemath-standard && SAGE_NUM_THREADS=16 tox -v -v -v -e sagepython-sagewheels-nopypi)'
(to be run after make build)
Depends on #33936
Component: refactoring
Author: Matthias Koeppe
Branch: ca9d6ce
Reviewer: Dima Pasechnik
Issue created by migration from https://trac.sagemath.org/ticket/32716