Trac #29367: tox.ini: Add tests for python2

These tests can be run by adding `-python2` to the tox environment,
such as
{{{
    tox -e local-homebrew-macos-standard-python2
    tox -e docker-ubuntu-bionic-standard-python2
}}}

Additionally, now `local` environments (introduced in #29104 for
homebrew) respect the environment variable `EXTRA_CONFIGURE_ARGS` that
was already available for `docker` environments.

URL: https://trac.sagemath.org/29367
Reported by: mkoeppe
Ticket author(s): Matthias Koeppe
Reviewer(s): Dima Pasechnik

tox.ini

@@ -11,6 +11,7 @@
 envlist =
     check_configure,
     {
+    {
     ### "docker" toxenvs copy sources from the source tree subject to the exclusions in
     ### the file ".dockerignore".  This should work out of non-clean source trees, and all
     ### "docker" toxenvs can be run in parallel.
@@ -64,15 +65,31 @@ envlist =
     #   $ tox -e local-homebrew-macos-standard -- ppl-clean ppl V=1
     #
     local-homebrew-macos
-   }-{minimal,standard,maximal},
+   }
+   -{###
+     ### Package factors:
+     ###
+     minimal,      # Install a minimal set of system packages that supports bootstrapping and compiling Sage.
+     standard,     # Install all known system packages equivalent to standard packages that have spkg-configure.m4
+     maximal       # Install all known system packages equivalent to standard/optional packages that have spkg-configure.m4
+    },
+   ###
    ### The "local-direct" toxenv passes the whole environment on to the sage build.
    ### Whatever is in PATH etc. will be used.
+   ###
    local-direct
+   }
+   -{###
+     ### Configuration factors:
+     ###
+     python2       # configure Sage to use Python 2 instead of the default (Python 3)
+    }
 
 skipsdist = true
 
 [testenv]
 passenv =
+                  EXTRA_CONFIGURE_ARGS
     docker:       EXTRA_DOCKER_BUILD_ARGS
     local:        MAKE
     local-direct: *
@@ -215,6 +232,16 @@ setenv =
     local-homebrew:   PATH={env:HOMEBREW}/bin:/usr/bin:/bin:/usr/sbin:/sbin
     local-homebrew:   SETENV=. .homebrew-build-env
 
+    #
+    # Configuration factors
+    #
+    python2:          CONFIG_CONFIGURE_ARGS_1=--with-python=2
+
+    #
+    # Resulting full configuration args, including EXTRA_CONFIGURE_ARGS from the user environment
+    #
+                      CONFIGURE_ARGS={env:CONFIG_CONFIGURE_ARGS_1:} {env:CONFIG_CONFIGURE_ARGS_2:} {env:EXTRA_CONFIGURE_ARGS:}
+
 # environment will be skipped if regular expression does not match against the sys.platform string
 platform =
     local-macos:   darwin
@@ -243,13 +270,13 @@ commands =
     # configure binfmt-support on the Docker host (works locally or remotely, i.e: using boot2docker)
     docker-{arm64,armhf}:  docker run --rm --privileged multiarch/qemu-user-static:register --reset
     docker:        docker build . -f {envdir}/Dockerfile                          \
-    docker:        --build-arg EXTRA_CONFIGURE_ARGS="{env:EXTRA_CONFIGURE_ARGS:}" \
+    docker:        --build-arg EXTRA_CONFIGURE_ARGS="{env:CONFIGURE_ARGS}" \
     docker:        --build-arg BASE_IMAGE={env:FULL_BASE_IMAGE_AND_TAG}           \
     docker-conda:  --build-arg USE_CONDARC="{env:CONDARC}"                        \
     docker:        {env:EXTRA_DOCKER_BUILD_ARGS:}
     # pathpy checksuite needs tox. #28728: gap fails its test suite.
     # linbox/cysignals testsuites fail.
-    local:         bash -c 'export PATH={env:PATH} && {env:SETENV} && ./bootstrap && ./configure --prefix={envdir}/local && make -k V=0 base-toolchain && SAGE_CHECK=yes SAGE_CHECK_PACKAGES="!cython,!r,!python3,!python2,!nose,!pathpy,!gap,!cysignals,!linbox" make -k V=0 {posargs:build}'
+    local:         bash -c 'export PATH={env:PATH} && {env:SETENV} && ./bootstrap && ./configure --prefix={envdir}/local {env:CONFIGURE_ARGS} && make -k V=0 base-toolchain && SAGE_CHECK=yes SAGE_CHECK_PACKAGES="!cython,!r,!python3,!python2,!nose,!pathpy,!gap,!cysignals,!linbox" make -k V=0 {posargs:build}'
 
 [testenv:check_configure]
 ## Test that configure behaves properly