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: Ie594fc55eb54d48d96157e31a319f0ff97afc9be
Closes-Bug:  #1801691

openstack/python-heatclient