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: Id73cfadea402d211a807b7f8df4dd374a624dece

tox.ini

 [tox]
-minversion = 3.1.1
+minversion = 3.18.0
 envlist = py3,pep8
 basepython = python3
 ignore_basepython_conflict = true
@@ -23,7 +23,7 @@ commands = {posargs}
 commands = python setup.py test --coverage --coverage-package-name=oslo_versionedobjects --testr-args='{posargs}'
 
 [testenv:docs]
-whitelist_externals = rm
+allowlist_externals = rm
 deps =
   -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
   -r{toxinidir}/doc/requirements.txt
@@ -32,7 +32,7 @@ commands =
   sphinx-build -W --keep-going -b html doc/source doc/build/html
 
 [testenv:releasenotes]
-whitelist_externals = rm
+allowlist_externals = rm
 deps = {[testenv:docs]deps}
 commands =
   rm -rf releasenotes/build