• Apparently sage-bootstrap-python using python 2.6 is broken already - https://groups.google.com/g/sage-support/c/GniJKQ0MaMY/m/DugFUprJAwAJ
• Platforms such as RHEL 6 and Centos 6 reached their EOL at the end of 2020; and Docker-based testing of centos-6 no longer works

In this ticket, we

• delete sage_bootstrap/compat/argparse.py
• Change ​http://doc.sagemath.org/html/en/developer/packaging.html#checksums and any other related documentation (last modified in Installation manual: Fix documentation of python prerequisite (sage-bootstrap-python) #31192)

This is on top of #31419 so that we have to change README only once.

Depends on #29890
Depends on #31419
Depends on #31318

CC: @fchapoton @dimpase @embray @kiwifb @jdemeyer @slel @jhpalmieri

Component: build

Author: Dima Pasechnik, Matthias Koeppe

Branch/Commit: 8cd0bcc

Reviewer: Matthias Koeppe, Dima Pasechnik

Issue created by migration from https://trac.sagemath.org/ticket/20023