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: I4ad1fbca83af0f0c89c3f18d38caa66ee735ea3e

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-minversion = 3.1.1
+minversion = 3.18.0
 envlist = py3,pypy,pep8
 ignore_basepython_conflict = true
 
@@ -23,7 +23,7 @@ commands =
 commands = {posargs}
 
 [testenv:docs]
-whitelist_externals =
+allowlist_externals =
   rm
 deps =
   {[testenv]deps}
@@ -57,7 +57,7 @@ import_exceptions =
   oslo_privsep._i18n
 
 [testenv:releasenotes]
-whitelist_externals =
+allowlist_externals =
   rm
 deps =
   -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}