Support minimal system Python version 3.6 (for venv)

[embray]

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

[mkoeppe]

comment:2

Related to the question of what Python versions are good is #26457 - "Do not depend on a patched Python"

[mkoeppe]

comment:3

Batch modifying tickets that will likely not be ready for 9.1, based on a review of the ticket title, branch/review status, and last modification date.

[mkoeppe]

Description changed:

--- 
+++ 
@@ -1,4 +1,4 @@
-With #29032 it is possible to use an existing Python 3.7 on the system to build and run Sage.  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.
+With #27824 it is possible to use an existing Python 3.7 on the system to build and run Sage.  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).
 

[mkoeppe]

Changed dependencies from #29032 to none

[mkoeppe]

comment:4

This would need rebasing

[mkoeppe]

Description changed:

--- 
+++ 
@@ -1,4 +1,6 @@
-With #27824 it is possible to use an existing Python 3.7 on the system to build and run Sage.  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.
+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).
 

[mkoeppe]

comment:5

Edited description to clarify that this is about the venv python3, not sage-system-python.

[mkoeppe]

Description changed:

--- 
+++ 
@@ -4,4 +4,13 @@
 
 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).
 
-I think this would be a nice-to-have, if not strictly necessary.  Maybe since Sage is less tested with 3.6, there could be a warning against using it.  But actually I'd be happy to keep running a Python 3.6 patchbot for a while until/unless it becomes untenable to support Python 3.6.
+This would be a nice-to-have, as it would speed up the build on various distributions that we still support: 
+- `centos-7` (3.6.8), `-8` (3.6.8), 
+- `fedora-26` (3.6.5, but has 3.7.0 too), `27` (3.6.6, but has 3.7.0 too), `-28` (3.6.5, but has 3.7.3 too), 
+- `opensuse-leap-15.0` (3.6.5), `opensuse-15.1 (3.6.9), opensuse-15.2 (3.6.10), 
+but not:
+- `debian-oldstable` (3.5.3),
+- `ubuntu-xenial` (3.5.2).
+(All this is according to https://repology.org/project/python/versions, not verified.
+
+Maybe since Sage is less tested with 3.6, there could be a warning against using it.

[mkoeppe]

Description changed:

--- 
+++ 
@@ -6,8 +6,8 @@
 
 This would be a nice-to-have, as it would speed up the build on various distributions that we still support: 
 - `centos-7` (3.6.8), `-8` (3.6.8), 
-- `fedora-26` (3.6.5, but has 3.7.0 too), `27` (3.6.6, but has 3.7.0 too), `-28` (3.6.5, but has 3.7.3 too), 
-- `opensuse-leap-15.0` (3.6.5), `opensuse-15.1 (3.6.9), opensuse-15.2 (3.6.10), 
+- `fedora-26` (3.6.5, but has 3.7.0 too), `-27` (3.6.6, but has 3.7.0 too), `-28` (3.6.5, but has 3.7.3 too), 
+- `opensuse-leap-15.0` (3.6.5), `-15.1` (3.6.9), `-15.2` (3.6.10), 
 but not:
 - `debian-oldstable` (3.5.3),
 - `ubuntu-xenial` (3.5.2).

[mkoeppe]

Changed branch from u/embray/build/configure/python3.6 to u/mkoeppe/build/configure/python3.6