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: Ie426b70156f777654d78bb3a3dada05f733abdc5

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-minversion = 2.0
+minversion = 3.18.0
 envlist = py3,pep8
 skipsdist = True
 
@@ -14,7 +14,7 @@ deps =
   -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
   -r{toxinidir}/test-requirements.txt
   -r{toxinidir}/requirements.txt
-whitelist_externals =
+allowlist_externals =
   rm
 commands = rm -f .testrepository/times.dbm
            # The --test-path is defined in .stestr.conf
@@ -52,7 +52,7 @@ commands = sphinx-build -W -b html doc/source doc/build/html
 basepython = python3
 envdir = {toxworkdir}/docs
 deps = {[testenv:docs]deps}
-whitelist_externals =
+allowlist_externals =
   rm
   make
 commands =