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.

Wallaby support officially the following runtimes [1]:
- Python 3.6
- Python 3.8

During Victoria Python 3.7 was used as the default runtime [2] however this
version isn't longer officially supported.

[1] https://governance.openstack.org/tc/reference/runtimes/wallaby.html#python-runtimes-for-wallaby
[2] https://governance.openstack.org/tc/reference/runtimes/victoria.html#python-runtimes-for-victoria

Change-Id: I3a98d1c258389968a892d6ccd067d3b8fb78a29a

tox.ini

 [tox]
 minversion = 3.1.1
-envlist = py38,pep8
+envlist = py3,pep8
 skipsdist = True
 ignore_basepython_conflict = true