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: I7b4606fd49ea7919cff56d819a2a5236ea3b1e73
diff --git a/tox.ini b/tox.ini
index d3e722a..ff50b9d 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,5 +1,5 @@
 [tox]
-minversion = 3.1
+minversion = 3.18.0
 envlist = pep8
 skipsdist = True
 ignore_basepython_conflict = True
@@ -22,7 +22,7 @@
 commands =
   sh ./tools/misc-sanity-checks.sh
   flake8
-whitelist_externals =
+allowlist_externals =
   sh
 
 [testenv:venv]