This makes the following changes to tox.ini:

• tox -e local-homebrew-macos-standard now uses python3 from homebrew.

  This attempts to reproduce the environment of https://groups.google.com/d/msg/sage-release/kU5M1QVuQQY/Zu10Q-CKAwAJ

• tox -e local-homebrew-macos-standard-python3_xcode uses /usr/bin/python3 (was the old default)

• tox -e local-homebrew-macos-standard-python3_pythonorg uses the Python 3.7 installation from the binary package from python.org

New toolchain factors: gcc_spkg, gcc_9.

Improved docker interface.

CC: @jhpalmieri @dimpase @orlitzky @kliem @vbraun

Component: porting

Author: Matthias Koeppe

Branch/Commit: b7bb418

Reviewer: Dima Pasechnik

Issue created by migration from https://trac.sagemath.org/ticket/29417