Added `./mach python-safety`, distinct from python-test so it doesn't have
to be run on every CI job - its errors may not depend on the area the push has changed.
Added the python/safety directory to ensure a different Pipfile is used, avoiding
conflicts with python-test.
Differential Revision: https://phabricator.services.mozilla.com/D1825
--HG--
extra : moz-landing-system : lando