Switch testing to Victoria testing runtime

Upating the testing template from ussuri to victoria
testing runtime
- https://governance.openstack.org/tc/reference/runtimes/victoria.html

Change-Id: Ifd65c0359509a4b59f3e876c42cef7cbf6f9ef93
diff --git a/tox.ini b/tox.ini
index c598d9e..bb5f0d7 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,6 +1,6 @@
 [tox]
 minversion = 3.1.1
-envlist = pep8,py36,py37
+envlist = pep8,py36,py38
 skipsdist = True
 ignore_basepython_conflict = True