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: Iaa02d12b545597a3b064f320ee33253300a5de1b

tox.ini

 [tox]
 minversion = 3.2.0
-envlist = py38,pep8
+envlist = py3,pep8
 basepython = python3
 ignore_basepython_conflict = true