Change minversion of tox to 3.18.0

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

Change-Id: Icf6ef4d9a0d619374c535c0bff7577d5af351886

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-minversion = 3.8.0
+minversion = 3.18.0
 envlist = docs,py38,pep8
 skipsdist = True
 ignore_basepython_conflict = True
@@ -15,7 +15,7 @@ setenv = VIRTUAL_ENV={envdir}
 deps = -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
        -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
-whitelist_externals = bash
+allowlist_externals = bash
                       sh
 passenv = http_proxy HTTP_PROXY https_proxy HTTPS_PROXY no_proxy NO_PROXY TOX_ENV_SRC_MODULES
 commands =
@@ -77,7 +77,7 @@ commands = sphinx-build -W -b html doc/source doc/build/html
 [testenv:pdf-docs]
 envdir = {toxworkdir}/docs
 deps = {[testenv:docs]deps}
-whitelist_externals =
+allowlist_externals =
   make
 commands =
   sphinx-build -W -b latex doc/source doc/build/pdf
@@ -144,7 +144,7 @@ deps =
 [testenv:requirements]
 deps =
   -egit+https://opendev.org/openstack/requirements#egg=openstack-requirements
-whitelist_externals = sh
+allowlist_externals = sh
 commands =
     sh -c '{envdir}/src/openstack-requirements/playbooks/files/project-requirements-change.py --req {envdir}/src/openstack-requirements --local {toxinidir} master'