Let's use openstack-python3-wallaby-jobs template.
Also update setup.cfg to be compatible with supported pythons.
flake8==2.6.2 and pyflakes==2.0.0 seems not to be compatible
with python3.8 on Ubuntu so the patch addresses this as well.
Change-Id: Ic09b2d836315d9a223bb8592d0e48ef475c56aa5