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: I3b687738c170dea60fca7fb2fe74fa5bbc37ab02

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-minversion = 3.1.1
+minversion = 3.18.0
 envlist = py38,pep8
 skipsdist = True
 # Automatic envs (pyXX) will only use the python version appropriate to that
@@ -10,7 +10,7 @@ ignore_basepython_conflict = True
 [testenv]
 basepython = python3
 usedevelop = True
-whitelist_externals =
+allowlist_externals =
   bash
   find
   rm
@@ -127,7 +127,7 @@ deps = {[testenv:docs]deps}
 commands =
   rm -rf api-ref/build
   sphinx-build -W -b html -d api-ref/build/doctrees api-ref/source api-ref/build/html
-whitelist_externals = rm
+allowlist_externals = rm
 
 [flake8]
 filename = *.py,app.wsgi