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: Ia9928d8fe69a6a3c593ff9b5846ef3123e7dda26

Author: likui

tox.ini

@@ -2,7 +2,7 @@
 
 [tox]
 envlist = py38,pep8
-minversion = 3.1.1
+minversion = 3.18.0
 skipsdist = True
 ignore_basepython_conflict = True
 
@@ -22,7 +22,7 @@ deps =
        -r{toxinidir}/test-requirements.txt
 commands = find . -type f -name "*.pyc" -delete
            stestr run {posargs}
-whitelist_externals = find
+allowlist_externals = find
                       rm
 
 [testenv:debug]