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