Change minversion of tox to 3.18.0

The patch bumps min version of tox to 3.18.0 in order to
replace whitelist_externals with allowlist_externals option.
https://github.com/tox-dev/tox/blob/master/docs/changelog.rst#v3180-2020-07-23

Signed-off-by: Takashi Natsume <takanattie@gmail.com>
Change-Id: If304d7c277850470448ca9bcd0f0bfd250ec001d

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 envlist = docs,py38,pep8
-minversion = 3.2.0
+minversion = 3.18.0
 skipsdist = True
 ignore_basepython_conflict = True
 
@@ -18,7 +18,7 @@ deps =
   -r{toxinidir}/requirements.txt
   -r{toxinidir}/test-requirements.txt
   hacking>=3.0.1,<3.1.0 # Apache-2.0
-whitelist_externals = sh
+allowlist_externals = sh
 commands =
   {toxinidir}/tools/pip_install_src_modules.sh "{toxinidir}"
   stestr run {posargs}
@@ -117,7 +117,7 @@ commands=
   {[testenv:bashate]commands}
   {[testenv:bandit]commands}
   {[testenv:genpolicy]commands}
-whitelist_externals =
+allowlist_externals =
   sh
   bash
 
@@ -149,7 +149,7 @@ commands = sphinx-build -W -b html doc/source doc/build/html
 [testenv:pdf-docs]
 envdir = {toxworkdir}/docs
 deps = {[testenv:docs]deps}
-whitelist_externals =
+allowlist_externals =
   make
 commands =
   sphinx-build -W -b latex doc/source doc/build/pdf
@@ -260,6 +260,6 @@ deps =
 [testenv:requirements]
 deps =
   -egit+https://opendev.org/openstack/requirements#egg=openstack-requirements
-whitelist_externals = sh
+allowlist_externals = sh
 commands =
     sh -c '{envdir}/src/openstack-requirements/playbooks/files/project-requirements-change.py --req {envdir}/src/openstack-requirements --local {toxinidir} master'