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: I24b41e47ba5b7d6094c05c338dbd1e4206d10f5c