In preparation for dropping support for Python 3.6 in Sage 9.4 (#30551), we issue a deprecation warning when configure accepts a system python 3.6. For example, on ubuntu-bionic Python 3.6 is the default, but Python 3.7 is also available, and users will want to consider using that by the configure option --with-python=....

To test: tox -e docker-ubuntu-bionic-standard -- config.status.

Depends on #31227
Depends on #31192

CC: @tobiasdiez @dimpase @slel @jhpalmieri

Component: build: configure

Author: Matthias Koeppe

Branch/Commit: 0fb8646

Reviewer: John Palmieri

Issue created by migration from https://trac.sagemath.org/ticket/31318