tox: add & document tox usage

This makes it easy to run all the tests against multiple versions
of Python.  We want to make sure Python 2.7 & 3.6+ work.

Change-Id: Ia7b16eb46a2aa7c240f03bb291987fa8cb215267
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/247174
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Mike Frysinger <vapier@google.com>