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[1] by allowlist_externals option.

[1] https://github.com/tox-dev/tox/blob/master/docs/changelog.rst#v3180-2020-07-23

Change-Id: I40e50948961cfbd6eb1dae1a05154c408cc15513

tox.ini

@@ -1,12 +1,12 @@
 [tox]
-minversion = 3.8
+minversion = 3.18.0
 envlist = linters,molecule
 skipsdist = True
 
 [testenv]
 usedevelop = True
 deps = -r{toxinidir}/test-requirements.txt
-whitelist_externals = bash
+allowlist_externals = bash
 setenv =
   ANSIBLE_ACTION_PLUGINS={toxinidir}/molecule/roles.galaxy/config_template/action:{toxinidir}/molecule/ansible_plugins/action
   ANSIBLE_CALLBACK_PLUGINS={toxinidir}/molecule/ansible_plugins/callback
@@ -47,7 +47,7 @@ commands =
 deps = -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
        -r{toxinidir}/doc/requirements.txt
 basepython = python3
-whitelist_externals = bash
+allowlist_externals = bash
 commands = bash -c ci-scripts/releasenotes_tox.sh
 
 [testenv:linters]