In Zed cycle testing runtime, we are targetting to drop the
python 3.6/3.7 support, project started adding python 3.8 as minimum,
example nova:
- 56b5aed08c/setup.cfg (L13)
Also moving zuul's job to `openstack-python3-jobs`.
Change-Id: I7997e1dc076ba4b9a385b96bfc97b7bf1c8aedd0