Update travis.yml to test Python 3.6-3.8
Update travis.yml to test Python 3.6-3.8

Django 3.x only supports Python 3.6, 3.7, 3.8 (#176)

Update travis.yml to begin testing these versions to identify issues and incompatibilities early
wfhio/tramcar,wfhio/tramcar,wfhio/tramcar