Use py3 as the default runtime for tox
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: Id917b883da6010f028a40be7acbe4e3a772f9408

openstack/cloudkitty,openstack/cloudkitty