Update README and Installation Guide (from Source) for Sage 9.1

[mkoeppe]

Follow-up from the updates in #29496. Related:

https://groups.google.com/d/msg/sage-release/aTSxEyE9Mrc/W_Vs8fIZAwAJ:

On Thursday, April 23, 2020 at 9:50:49 AM UTC-7, kcrisman wrote:
Built from scratch on OS X 10.11.6, using ./configure and then make. I suggest that https://doc.sagemath.org/html/en/installation/source.html is incorrect in suggesting that Py3 is necessary to build Sage.

You're right, this needs to be updated. Python 2 also works.

Related:

• Update README.md and Installation Manual for 9.1, integrate instructions for Windows (WSL and Cygwin) in manuals #25206 Windows-specific updates
• Add script package _recommended and generate "recommended system packages" info #29557 Add script package _recommended and generate "recommended system packages" info
• Autogenerated parts of installation guide #29558 homebrew section

Depends on #29558

CC: @kcrisman @dimpase @jhpalmieri @egourgoulhon @kiwifb

Component: documentation

Author: John Palmieri

Branch/Commit: 26a802e

Reviewer: Matthias Koeppe

Issue created by migration from https://trac.sagemath.org/ticket/29556

[jhpalmieri]

Branch: u/jhpalmieri/readme

[jhpalmieri]

comment:7

Here are some minimal changes. I think we should also add a homebrew section to the file source.rst. I can work on that, but should it be on this ticket or another one?

---

New commits:

3e583a6

trac 29556: update installation instructions

[jhpalmieri]

Commit: 3e583a6

[jhpalmieri]

comment:8

I'm putting the proposed homebrew section at #29558.

[mkoeppe]

comment:9

There are also some links to the wiki that point to outdated trash pages.

[dimpase]

comment:10

I'd say **python**: Python >= 3.6, or Python 2.7 (deprecated) - we do not want to give an impression we commit to Python 2 in any way.

[mkoeppe]

comment:11

The precise list of versions that we claim to support for sage-system-python can be found in build/tox.ini: py26, py27, py33, py34, py35, py36, py37, py38

[mkoeppe]

Dependencies: #29558

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1. New commits:

26a802e

trac 29556: allowable versions of system Python

[sagetrac-git]

Changed commit from 3e583a6 to 26a802e

[mkoeppe]

Author: John Palmieri

[mkoeppe]

Reviewer: Matthias Koeppe

[vbraun]

Changed branch from u/jhpalmieri/readme to 26a802e