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: Ibd1226b0e72ec374d36a90a826b8cc6c6b57d1f2

tox.ini

 [tox]
 minversion = 3.2.0
-envlist = py38,pep8
+envlist = py3,pep8
 ignore_basepython_conflict = True
 
 [testenv]