Update minversion of tox
Update minversion of tox

The minimum version since when tox accepts having inline comments
in deps is 3.9.0

https://tox.readthedocs.io/en/latest/changelog.html#v3-9-0-2019-04-17

Change-Id: Ibe03e0aeac8bab767e9897b9c34d9a954a0d678e

openstack/bifrost,openstack/bifrost