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: Ifb8d4f589c2fb8fa2adc0372d1d54077afe5b32d

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-minversion = 3.1.0
+minversion = 3.18.0
 envlist = py3,pep8
 skipsdist = True
 # this allows tox to infer the base python from the environment name
@@ -23,7 +23,7 @@ deps = -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/
 commands =
     find . -type f -name "*.pyc" -delete
     stestr run --serial --slowest {posargs}
-whitelist_externals = find
+allowlist_externals = find
 
 [testenv:integration]
 setenv = {[testenv]setenv}
@@ -68,7 +68,7 @@ commands =
 # This environment is called from CI scripts to test and publish
 # the API Ref to docs.openstack.org.
 #
-whitelist_externals = bash
+allowlist_externals = bash
                       rm
 deps =
     -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}