Fix outedated tox minversion

allowlist_externals is supported since 3.18.0 .

Change-Id: Iecd5175e5081b4f7ee66449a623064632133359d

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 envlist = pep8,eslint,karma-local,docs-local,releasenotes,lower-constraints-local
-minversion = 2.3.2
+minversion = 3.18.0
 
 [testenv]
 usedevelop = True
@@ -53,7 +53,7 @@ commands =
   {[testenv]commands}
 
 [testenv:eslint]
-Allowlist_externals =
+allowlist_externals =
   npm
 commands =
   npm install
@@ -63,14 +63,14 @@ commands =
 # from master branch into python3.x environment for testing javascripts.
 # Horizon from master is needed to be cloned into ../horizon on both local and CI.
 [testenv:karma]
-Allowlist_externals =
+allowlist_externals =
   {[testenv:eslint]whitelist_externals}
 commands =
   npm install
   npm run test
 
 [testenv:karma-local]
-Allowlist_externals =
+allowlist_externals =
   {[testenv:eslint]whitelist_externals}
 commands =
   {[testenv:karma]commands}