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: Ie214cbc11b37dd95a3f020c5dca725cbb531bf9c

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-minversion = 3.2.0
+minversion = 3.18.0
 envlist = py38,pep8
 skipsdist = True
 ignore_basepython_conflict = True
@@ -17,7 +17,7 @@ deps =
   -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
   -r{toxinidir}/requirements.txt
   -r{toxinidir}/test-requirements.txt
-whitelist_externals = sh
+allowlist_externals = sh
 commands =
   stestr run {posargs}
 
@@ -58,14 +58,14 @@ 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
   make -C doc/build/pdf
 
 [testenv:api-ref]
-whitelist_externals = rm
+allowlist_externals = rm
 deps =
   -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
   -r{toxinidir}/requirements.txt
@@ -79,7 +79,7 @@ deps =
   -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
   -r{toxinidir}/requirements.txt
   -r{toxinidir}/doc/requirements.txt
-whitelist_externals = rm
+allowlist_externals = rm
 commands =
   rm -rf api-ref/build
   sphinx-build -W -b linkcheck api-ref/source api-ref/build/linkcheck