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: I18820238e370fbd0836995a281b607da90871ef6

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-minversion = 3.1.1
+minversion = 3.18.0
 skipsdist = True
 envlist = py,pep8,packaging,sanity
 requires =
@@ -26,7 +26,7 @@ commands = {posargs}
 deps =
   -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
   -r{toxinidir}/doc/requirements.txt
-whitelist_externals =
+allowlist_externals =
   rm
 commands =
   rm -rf doc/build
@@ -80,7 +80,7 @@ commands =
   sh -c "ansible-playbook --syntax-check  playbooks/*.yaml"
   ansible localhost -m tripleo.repos.get_hash -a "release=master os_version=centos8"
 
-whitelist_externals =
+allowlist_externals =
   sh
 
 [testenv:molecule]