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: I138a808e799e4969af8542346289737c544a3c5e

tox.ini

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