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: I1d65fb9d8d302ff1e3f33e92b668b2241741e7e0

tox.ini

 [tox]
-minversion = 3.2.0
+minversion = 3.18.0
 envlist = py3,pep8,docs
 ignore_basepython_conflict = true
 
@@ -28,7 +28,7 @@ commands = {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 -fr doc/build
   sphinx-build -W --keep-going -b html doc/source doc/build/html
@@ -37,7 +37,7 @@ commands =
 commands = python setup.py test --coverage --coverage-package-name=oslo_limit --testr-args='{posargs}'
 
 [testenv:releasenotes]
-whitelist_externals = rm
+allowlist_externals = rm
 deps =
   -r{toxinidir}/doc/requirements.txt
 commands =