sage_bootstrap: Remove purported Python 2.6 support

[vbraun]

• 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

[karandesai-96]

comment:1

I changed the directory to SAGE_ROOT/build/sage_bootstrap and checked out contents :
`
.
├── cksum.py
├── cmdline.py
├── compat.py
├── config.py
├── download.py
├── env.py
├── init.py
├── levenshtein.py
├── logger.py
├── mirror_list.py
├── package.py
├── README
├── stdio.py
├── tarball.py
└── updater.py

`

There is no subdirectory named "compat".

[embray]

comment:2

This can also help with #9536 since Python 2.7+ supports the --no-user-cfg option to setup.py/setup.cfg, which Python 2.6 does not (just thought of this since it's an issue I looked at recently).

[vbraun]

Dependencies: #19984

[slel]

comment:5

See #28000 for complete removal of Python 2 support.

[embray]

comment:6

I'm pretty sure Python 2.6 support is completely removed from sagelib itself, just not from sage_bootstrap.

[dimpase]

comment:7

should we clean up sage_bootstrap in this respect here? (this would mean to require system's Python 2.7 or better for building)

[slel]

comment:8

Removing the ability to bootstrap from Python 2.6 probably does
not need to happen before we remove Python 2 support in sagelib.

[dimpase]

comment:9

From the brief look at the code, this is a change that may be done now, and after all noone must Python 2.6 now.

[kiwifb]

comment:10

It's the default python on redhat/centos 6 and possibly some ancient debian. I'll note that redhat/centos 6 have access to newer python but someone with admin privileges and knowledge of software collections is needed.

[dimpase]

comment:12

I'll make an experimental branch to remove vendored argparse

[dimpase]

Author: Dima Pasechnik

[dimpase]

Commit: 2d0891d

[dimpase]

Branch: u/dimpase/build/unvendorargparse

[dimpase]

New commits:

2d0891d

remove vendored argparse

[dimpase]

Changed dependencies from #19984 to none

[dimpase]

comment:14

tests run on https://github.com/dimpase/sage/pull/11

[mkoeppe]

comment:16

Also need to update build/tox.ini

[sagetrac-git]

Changed commit from 2d0891d to f734c8e

[sagetrac-git]

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

f734c8e

remove py26 from tox

[sagetrac-git]

Changed commit from 0f3d369 to 86c05e6

[sagetrac-git]

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

5e810dd	build/pkgs/python3: Update to 3.9.2
60d6433	Merge #31419
06873c2	README.md: Update Python requirement for build

[sagetrac-git]

Changed commit from 86c05e6 to 06873c2

[mkoeppe]

Changed dependencies from #29890 to #29890, #31419

[mkoeppe]

Changed reviewer from Matthias Koeppe to Matthias Koeppe, ...

[dimpase]

comment:39

should we also remove py33 from build/tox.ini ?

[sagetrac-git]

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

c948b02

build/tox.ini: Remove py33, add py39, py310

[sagetrac-git]

Changed commit from 06873c2 to c948b02

[dimpase]

comment:41

OK, this works

[dimpase]

Changed reviewer from Matthias Koeppe, ... to Matthias Koeppe, Dima Pasechnik

[mkoeppe]

comment:42

Thanks.

[vbraun]

comment:43

Merge conflict

[sagetrac-git]

Changed commit from c948b02 to b1fe929

[sagetrac-git]

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

92533b8	Merge tag '9.3.beta8' into t/20023/build/unvendorargparse
0cac965	build/pkgs/ecl: Update to 21.2.1, remove patches included in new version
6a62484	delete all merged upstream patches: MR 210, MR 231
b1fe929	Merge #31336

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1. This was a forced push. New commits:

6ef52d6	build/pkgs/python3/spkg-configure.m4: Issue a deprecation warning for system python 3.6
cf55561	Merge branch 't/31192/installation_manual__fix_documentation_of_python_prerequisite__sage_bootstrap_python_' into t/31318/python3_spkg_configure_m4__add_deprecation_warning_regarding_system_python_3_6
0fb8646	README.md, src/doc/en/installation/source.rst: Mention/update version range for python3
8cd0bcc	Merge #31318

[sagetrac-git]

Changed commit from b1fe929 to 8cd0bcc

[mkoeppe]

Changed dependencies from #29890, #31419 to #29890, #31419, #31318

[vbraun]

Changed branch from u/mkoeppe/build/unvendorargparse to 8cd0bcc