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: I1976e24792d506003962cf970cd59b8d47891f4a

openstack/ironic-lib