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: I1f1c8e6aaa3191794e2281be8eb47a119e4fc7ab

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-minversion = 3.1.1
+minversion = 3.18.0
 envlist = py38,pep8
 skipsdist = True
 # Automatic envs (pyXX) will only use the python version appropriate to that
@@ -18,7 +18,7 @@ setenv = VIRTUAL_ENV={envdir}
          NOSE_OPENSTACK_SHOW_ELAPSED=1
          PYTHONDONTWRITEBYTECODE=1
          DJANGO_SETTINGS_MODULE=designatedashboard.settings
-whitelist_externals = find
+allowlist_externals = find
 deps = -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
        -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
@@ -48,7 +48,7 @@ commands = sphinx-build -W -b html -d doc/build/doctrees doc/source doc/build/ht
 [testenv:pdf-docs]
 deps = -r{toxinidir}/doc/requirements.txt
 envdir = {toxworkdir}/docs
-whitelist_externals =
+allowlist_externals =
   make
 commands =
   sphinx-build -W -b latex doc/source doc/build/pdf