For Sage 9.2: Remove Python 2 support from the build system and CI scripts

[mkoeppe]

The ticket makes it an error (with a clear message) when ./configure --with-python=2 is invoked.

checking Python version to install... configure: error: the only allowed value for --with-python is 3.  Support for Python 2 has been removed in Sage 9.2.

The ticket keeps variables such as SAGE_PYTHON_VERSION and SAGE_PYTHON3 unchanged, for compatibility.

Also, to clarify in light of the discussion in https://groups.google.com/d/msg/sage-devel/fgsSsJmVVXo/ZIcJq9x3AAAJ, this ticket does not affect which versions of Python are sufficient as sage-system-python (this is still all versions listed in build/tox.ini, which includes various 2.x and 3.x).

Part of meta-ticket #29141.

Depends on #29633

CC: @dimpase @orlitzky @embray @fchapoton @jhpalmieri @vbraun

Component: build

Author: Matthias Koeppe

Branch/Commit: ff41817

Reviewer: Michael Orlitzky

Issue created by migration from https://trac.sagemath.org/ticket/29669

[mkoeppe]

Dependencies: #29633

[mkoeppe]

Branch: u/mkoeppe/for_sage_9_2__remove_python_2_support_from_the_build_system

[sagetrac-git]

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

96a5e76

Remove python2 from tox and [GitHub](../wiki/GitHub) CI scripts

[sagetrac-git]

Commit: 96a5e76

[mkoeppe]

Author: Matthias Koeppe

[orlitzky]

Reviewer: Michael Orlitzky

[orlitzky]

comment:7

This was never quite right,

case "$with_python" in
     3*) SAGE_PYTHON_VERSION=3;;

because it accepts --with-python=3.9 and ignores everything after the 3 (it should throw an error in my opinion). Not a big deal now since it will be deleted soon.

There's a lot of other python-2.x stuff to be removed in the build system, scripts, docs, and SPKGs, but I think you are aware of that and plan to deal with it later? If so this is fine as a first step.

[mkoeppe]

comment:8

Replying to @orlitzky:

This was never quite right,

case "$with_python" in
     3*) SAGE_PYTHON_VERSION=3;;

because it accepts --with-python=3.9 and ignores everything after the 3 (it should throw an error in my opinion).

Good point. Let me make this a warning actually.

[sagetrac-git]

Changed commit from 96a5e76 to ff41817

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1 and set ticket back to needs_review. New commits:

ea7df3e	configure.ac: Remove message 'Checking for Python version to install...'
ff41817	configure.ac: Warn on 'configure --with-python=3.x'

[mkoeppe]

comment:10

Replying to @orlitzky:

There's a lot of other python-2.x stuff to be removed in the build system, scripts, docs, and SPKGs, but I think you are aware of that and plan to deal with it later? If so this is fine as a first step.

Yes, there's a lot to be done, and there is a meta-ticket for that: #29141.

[mkoeppe]

comment:13

Thanks!

[vbraun]

Changed branch from u/mkoeppe/for_sage_9_2__remove_python_2_support_from_the_build_system to ff41817