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: I686257fe9ffaefde4d2cdca8ff60a83b908f3b6b

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-minversion = 3.1.1
+minversion = 3.18.0
 envlist = py38,pep8
 skipsdist = True
 ignore_basepython_conflict = true
@@ -53,7 +53,7 @@ commands = {toxinidir}/tools/cover.sh {posargs}
 deps =
   -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
   -r{toxinidir}/doc/requirements.txt
-whitelist_externals = rm
+allowlist_externals = rm
 commands =
   rm -rf doc/build
   sphinx-build -W -b html doc/source doc/build/html
@@ -62,7 +62,7 @@ commands =
 deps={[testenv:docs]deps}
 usedevelop = True
 
-whitelist_externals =
+allowlist_externals =
   make
 commands =
   sphinx-build -W -b latex doc/source doc/build/pdf