Bump tox minversion to 3.18.0

Since tox 3.18.0, the whitelist_externals option has been deprecated in
favor of the new allow_list_externals option[1].

This change bumps the minversion of tox so that we can replace
the deprecated option.

[1] https://github.com/tox-dev/tox/blob/master/docs/changelog.rst#v3180-2020-07-23

Change-Id: I76f328b8bed1338ab5496d21e54a4b0ff9251147

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 envlist = py39,pep8
-minversion = 2.0
+minversion = 3.18.0
 skipsdist = True
 
 [testenv]
@@ -35,7 +35,7 @@ warnerror = True
 setenv =
   OS_TEST_PATH = ./glanceclient/tests/functional/v2
   OS_TESTENV_NAME = {envname}
-whitelist_externals =
+allowlist_externals =
     bash
 commands =
     bash tools/fix_ca_bundle.sh