Use py3 as the default runtime for tox

Moving on py3 as the default runtime for tox to avoid to update
this at each new cycle.

Change-Id: Ie18dd47beef0d15e168c3284c4cf0d3564467eff
diff --git a/tox.ini b/tox.ini
index a4f25d3..950cdbc 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,6 +1,6 @@
 [tox]
 minversion = 3.18.0
-envlist = pep8,py36,py38
+envlist = pep8,py3
 skipsdist = True
 ignore_basepython_conflict = True