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: Ice25307cb403768f3a7909395773ee38154f46ec

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 envlist = py39,pep8,pylint
-minversion = 3.1.1
+minversion = 3.18.0
 skipsdist = True
 
 [testenv]
@@ -14,7 +14,7 @@ usedevelop = True
 deps = -c{env:UPPER_CONSTRAINTS_FILE:https://opendev.org/openstack/requirements/raw/branch/master/upper-constraints.txt}
        -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
-whitelist_externals =
+allowlist_externals =
   sh
   find
 commands =
@@ -83,7 +83,7 @@ setenv =
   OS_ROOTWRAP_DAEMON_CMD=sudo {envdir}/bin/neutron-rootwrap-daemon {envdir}/etc/neutron/rootwrap.conf
   OS_FAIL_ON_MISSING_DEPS=1
   OS_NEUTRON_PATH={env:OS_NEUTRON_PATH:/home/zuul/src/opendev.org/openstack/neutron}
-whitelist_externals =
+allowlist_externals =
   sh
   cp
   sudo
@@ -103,7 +103,7 @@ commands =
   neutron-db-manage --subproject neutron-fwaas --database-connection sqlite:// check_migration
   {[testenv:genconfig]commands}
   {[testenv:genpolicy]commands}
-whitelist_externals = sh
+allowlist_externals = sh
 
 [testenv:cover]
 setenv = VIRTUAL_ENV={envdir}
@@ -123,7 +123,7 @@ deps = -r{toxinidir}/doc/requirements.txt
 
 [testenv:docs]
 deps = -r{toxinidir}/doc/requirements.txt
-whitelist_externals =
+allowlist_externals =
   rm
 commands =
   rm -rf doc/source/contributor/api
@@ -132,7 +132,7 @@ commands =
 [testenv:pdf-docs]
 envdir = {toxworkdir}/docs
 deps = {[testenv:docs]deps}
-whitelist_externals =
+allowlist_externals =
   rm
   make
 commands =