Update default envlist

Python 3.8 support was already removed so it should no longer be in
the default envlist. Also use unversioned target instead of versioned
ones so that we don't have to update the envlist when supported python
versions are updated.

Change-Id: I72f559b1e58ab3009585b2e03898d5fc1cf49e09

Author: kajinamit

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 3.18.0
-envlist = docs,py38,pep8
+envlist = docs,py3,pep8
 skipsdist = False
 ignore_basepython_conflict = True