It was suggested (https://groups.google.com/g/sage-devel/c/cSsAsPuVnxg/m/uWEySfvWBQAJ) that testing of new Python versions (such as the upcoming 3.10) could be done via pyenv instead of test tickets (such as #30766) that update our python3 spkg.

In this ticket, we add supporting infrastructure for this in tox.ini.

• https://github.com/pyenv/pyenv
• https://tox.readthedocs.io/en/latest/plugins.html tox-pyenv

Upstream issue:

• pythons built with pyenv's python-build leak CFLAGS/LDFLAGS into user packages via sysconfig pyenv/pyenv#2204

• python-build breaks when MAKE is set as we document it for Sage (`export MAKE="make -j15"):

  /usr/local/Cellar/pyenv/2.3.0/plugins/python-build/bin/python-build: line 789: make -j15: command not found
  

Depends on #29285

Upstream: Reported upstream. No feedback yet.

CC: @dimpase @orlitzky

Component: porting

Issue created by migration from https://trac.sagemath.org/ticket/32531