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: I382ca56a7f04aba92fbff060e28df6814712a153

tox.ini

 [tox]
-minversion = 3.2.0
+minversion = 3.18.0
 envlist = py3,pep8
 ignore_basepython_conflict = True
 
 [testenv]
 basepython = python3
-whitelist_externals =
+allowlist_externals =
   find
 deps =
   -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
@@ -24,7 +24,7 @@ commands =
 commands = {posargs}
 
 [testenv:docs]
-whitelist_externals = rm
+allowlist_externals = rm
 deps =
   -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
   -r{toxinidir}/doc/requirements.txt
@@ -52,7 +52,7 @@ exclude=.venv,.git,.tox,dist,doc,*lib/python*,*egg,build
 import_exceptions =
 
 [testenv:releasenotes]
-whitelist_externals =
+allowlist_externals =
   rm
 deps = {[testenv:docs]deps}
 commands =