python3 spkg-configure.m4: Add deprecation warning regarding system python 3.6

[mkoeppe]

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

[mkoeppe]

Dependencies: #31227

[mkoeppe]

Changed dependencies from #31227 to #31227, #31192

[mkoeppe]

Branch: u/mkoeppe/python3_spkg_configure_m4__add_deprecation_warning_regarding_system_python_3_6

[mkoeppe]

Author: Matthias Koeppe

[mkoeppe]

Last 10 new commits:

df17ad4	tox.ini: Add configuration factors for specific homebrew python3.x
914ea58	build/pkgs/python3/spkg-configure.m4: Get rid of stray AC_SUBST
986739b	m4/sage_check_python_for_venv.m4: Fix reason
0b3e70d	build/pkgs/python3/spkg-configure.m4: If PYTHON_FOR_VENV is configured to build multiarch extensions, set SAGE_ARCHFLAGS to disable it
372adcb	.github/workflows/tox.yml: Update xcode versions
fc8b676	SAGE_CHECK_PYTHON_FOR_VENV: Rework with less nesting
6ef52d6	build/pkgs/python3/spkg-configure.m4: Issue a deprecation warning for system python 3.6
4265656	src/doc/en/installation/source.rst: Fix documentation of python prerequisite (sage-bootstrap-python)
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

[mkoeppe]

Commit: 0fb8646

[jhpalmieri]

comment:7

This seems okay, but I am not going to be able to test it very effectively: I don't have easy access to a system which has Python 3.6 as its latest version, and I don't want to set one up.

[jhpalmieri]

Reviewer: John Palmieri

[jhpalmieri]

comment:9

Okay, looks like it works the way it's supposed to.

[mkoeppe]

comment:10

Thank you!

[vbraun]

Changed branch from u/mkoeppe/python3_spkg_configure_m4__add_deprecation_warning_regarding_system_python_3_6 to 0fb8646