python_requires >= 3.8 during Yoga

gmann has been helpfully proposing patches to change the versions of Python we're testing against in Yoga. I've suggested that we might want to bump 'python_requires' in 'setup.cfg' to indicate that we no longer support any version of Python before 3.8 [1]. As gmann has noted, doing so would mean nova would no longer be installable on Python 3.6 or 3.7 and there has been a small bit of back and forth on the pros and cons of this. I'm wondering what other people's thoughts on this are. Is this something we should be doing? Should we do it for libraries too or just services? Do we ever want to do this? Thoughts, please! Stephen [1] https://review.opendev.org/c/openstack/nova/+/819194/comment/72ecf24f_2bd292...