Changed minversion in tox to 3.18.0
Changed minversion in tox to 3.18.0

The patch bumps min version of tox to 3.18.0 in order to
replace tox's whitelist_externals by allowlist_externals option:
https://github.com/tox-dev/tox/blob/master/docs/changelog.rst#v3180-2020-07-23

Change-Id: Icba1345ce6ab9819ad5683c3227573fb961c7f0d

openstack/python-tripleoclient