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: Idb0f7ff31a8fa788c940d70eddd1e7218691a31d

tox.ini

 [tox]
 minversion = 3.2.0
-envlist = py37,pep8,docs
+envlist = py3,pep8,docs
 ignore_basepython_conflict = true
 
 [testenv]