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: I143ef78dec2ffff6b1c309036d50533591c20171

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-minversion = 3.9.0
+minversion = 3.18.0
 skipsdist = True
 envlist = py3,pep8,functional-py38
 ignore_basepython_conflict=true
@@ -28,7 +28,7 @@ usedevelop = False
 deps =
     -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
     -r{toxinidir}/doc/requirements.txt
-whitelist_externals = bash
+allowlist_externals = bash
 commands =
   sphinx-build -W -b html -d api-ref/build/doctrees api-ref/source api-ref/build/html
 
@@ -105,7 +105,7 @@ commands =
   sphinx-build -W -b html doc/source doc/build/html
 
 [testenv:pdf-docs]
-whitelist_externals = make
+allowlist_externals = make
 deps = {[testenv:docs]deps}
 commands =
   sphinx-build -b latex doc/source doc/build/pdf