Update min tox version to 2.0
Update min tox version to 2.0

The commands used by constraints need at least tox 2.0.  Update to
reflect reality, which should help with local running of constraints
targets.

Change-Id: I413110b14dfe294c8a51c1dc23232bd4f09619dd
Closes-Bug:  #1802030

openstack/python-barbicanclient