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: Ida8f5339d8525bca66cb7a505fee45fc0bc39b7c

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-minversion = 3.1.1
+minversion = 3.18.0
 envlist = py3,pep8
 skipsdist = True
 ignore_basepython_conflict = True
@@ -37,7 +37,7 @@ commands =
 [testenv:docs]
 deps = -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
        -r{toxinidir}/doc/requirements.txt
-whitelist_externals =
+allowlist_externals =
   rm
 commands =
   rm -fr doc/build
@@ -45,7 +45,7 @@ commands =
 
 [testenv:releasenotes]
 deps = {[testenv:docs]deps}
-whitelist_externals =
+allowlist_externals =
   rm
 commands =
   rm -rf releasenotes/build