bump py37 to py38 in tox.ini

in 'victoria' cycle, we should test py38 by default.

Change-Id: Id1b69b2e4fa56ade670a72f0775690beb09bf89d
diff --git a/tox.ini b/tox.ini
index efc6c97..4591a45 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,6 +1,6 @@
 [tox]
 minversion = 3.1.1
-envlist = py37,pypy,pep8
+envlist = py38,pypy,pep8
 skipsdist = True
 ignore_basepython_conflict = True