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: I7482b0cfa02e0ac5dfb9813e1eb7c4e4d032b8ac

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-minversion = 3.1.1
+minversion = 3.18.0
 envlist = py3,pep8
 ignore_basepython_conflict = true
 
@@ -24,7 +24,7 @@ commands =
 commands = {posargs}
 
 [testenv:docs]
-whitelist_externals =
+allowlist_externals =
   rm
 deps =
   {[testenv]deps}
@@ -48,7 +48,7 @@ exclude=.venv,.git,.tox,dist,doc,*lib/python*,*egg,build
 import_exceptions =
 
 [testenv:releasenotes]
-whitelist_externals =
+allowlist_externals =
   rm
 deps =
   -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}