Bump minimum tox version

The allowlist_externals alias was added in 3.18.0 [1].

[1] https://tox.wiki/en/3.18.0/changelog.html

Change-Id: I95d0ab47444cd7b39a228b7a60d078d8f6589a0f

tox.ini

 [tox]
-minversion = 3.1
+minversion = 3.18.0
 envlist = py3,pep8
 ignore_basepython_conflict = True