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: I1cc2c6fa8577e50b7044e0105957a1d42580c6ca

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 envlist = pep8,py36,py37
-minversion = 3.1.1
+minversion = 3.18.0
 skipsdist = True
 ignore_basepython_conflict = True
 
@@ -36,7 +36,7 @@ commands =
   coverage html --omit '.tox/cover/*' -d 'cover/htmlcov'
 
 [testenv:eslint]
-whitelist_externals = npm
+allowlist_externals = npm
 commands =
   npm install
   npm run postinstall
@@ -45,12 +45,12 @@ commands =
 [testenv:karma]
 # NOTE(shu-mutou): Until PhantomJS setup get reliable, we use
 # Chromium for JS test. And npm can't launch Chromium via tox.
-#whitelist_externals = npm
+#allowlist_externals = npm
 #commands =
 #  npm install
 #  npm run postinstall
 #  npm run test
-whitelist_externals = echo
+allowlist_externals = echo
 commands =
   echo "npm can't launch Chromium via tox."
   echo "nexecute `npm run test`"