Use Python 3.8 as the default runtime for tox

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.

Moving on the most recent officially supported runtime as the default runtime
for tox.

[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: I93b58f133efe115cc954932ebca887d52f9edfac

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 3.1.0
-envlist = py37,pep8
+envlist = py38,pep8
 skipsdist = True
 # Automatic envs (pyXX) will only use the python version appropriate to that
 # env and ignore basepython inherited from [testenv] if we set