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.

[git]

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

​96a5e76

Remove python2 from tox and GitHub CI scripts

[mjo]

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]

Replying to mjo:

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.

[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]

Replying to mjo:

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]

Thanks!