We should support Python 3.9 (expected to be released 2020-10-05) in Sage 9.2.

The present ticket is for the patches necessary to support Python 3.9 in addition to 3.7 and 3.8.

It does not include the actual upgrade to Python 3.9 - to test, merge #30589.

• Python 3.9 release schedule
• What's new in Python 3.9
• Adopt the “time window-based” policy for support of Python versions from NEP 29 #30384 Adopt the “time window-based” policy for support of Python versions from NEP 29

Depends on #27754
Depends on #28012
Depends on #30590
Depends on #30053
Depends on #30595
Depends on #30576
Depends on #29500
Depends on #30546

CC: @antonio-rojas @embray @kiwifb @kliem @timokau @isuruf @jhpalmieri @mkoeppe @slel @tobihan @tscrim @jdemeyer @dimpase

Component: packages: standard

Keywords: upgrade, python

Author: Matthias Koeppe

Branch: 9d06080

Reviewer: John Palmieri, Dima Pasechnik

Issue created by migration from https://trac.sagemath.org/ticket/30184