With #27824 it is possible to create a venv over an existing Python 3.7 instead of building the python3 spkg.

This is designed to work with Python 3.7 (and < 3.8) since that's what's currently used by sage-the-distribution and is best-tested.

However, it turns out an existing Python 3.6 can work just fine as the minimum Python version with a few small fixes (sage-the-distribution used Python 3.6 until last year, so the degree to which we diverged from supporting it was minimal).

This would be a nice-to-have, as it would speed up the build on various distributions that we still support:

• centos-8 (3.6.8),
• fedora-26 (3.6.5, but has 3.7.0 too), https://github.com/mkoeppe/sage/runs/743789829 - clean doctests, but exceeds 6h
• fedora-27 (3.6.6, but has 3.7.0 too), https://github.com/mkoeppe/sage/runs/743789883 - clean except for sympow, exceeds 6h
• fedora-28 (3.6.5, but has 3.7.3 too), https://github.com/mkoeppe/sage/runs/743789896 - with gcc_spkg, build errors for gmpy2, cython, pynac
• opensuse-leap-15.0 (3.6.5), -15.1 (3.6.9), -15.2 (3.6.10),
  but not (because python3 is too old):
• debian-stretch (oldstable) (3.5.3),
• ubuntu-xenial (3.5.2)
  and not (because a dependency of python3 is too old):
• centos-7 (3.6.8): sqlite is too old (https://github.com/mkoeppe/sage/runs/743789990)

(All this is according to https://repology.org/project/python/versions as of 2020-06-05, not verified.)

Maybe since Sage is less tested with 3.6, there could be a warning against using it.

See also: #27754 (Upgrade: Python 3.8.x)

CC: @jhpalmieri @dimpase @kiwifb @orlitzky

Component: python3

Author: Erik Bray, Matthias Koeppe

Branch: abb5607

Reviewer: Matthias Koeppe, Michael Orlitzky

Issue created by migration from https://trac.sagemath.org/ticket/29033