MacOS users may need to set OBJC_DISABLE_INITIALIZE_FORK_SAFETY=TRUE
(#27086) to prevent tests from hanging. If set, also pass the environment
variable to `tox`.
PYTHONPATH is required for passing through test modules that don't
already exist within the Django tests/ directory.
DISPLAY is required for testing selenium on systems that use Xvfb.
HOME is where the Vagrant djangobox stores geoip data, but it could be
useful for other tox commands that require access to data outside of
Django.
