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: I03700a8ee7e5d38ec0ab5933ef277c422a228ef3
diff --git a/tox.ini b/tox.ini
index 5ae7b00..406cee4 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,6 +1,6 @@
 [tox]
 minversion = 3.1.1
-envlist = py37,py36,pypy,pep8
+envlist = py3,pypy,pep8
 skipsdist = True
 ignore_basepython_conflict = True