Describe your feature request

Some CI systems have base images for testing. Using something like python:latest would grab Python 3.9. Since there are no Python 3.9 wheels for Ray, there will be many silent and hard to debug failures in CI systems (speaking from experience here). Also, 3.9 is also the new stable version of Python, and Ray should be supporting that as well.

There may have to be a discussion amongst the Ray team how far many older versions they want to build wheels for. It would be nice if the Ray team could support 0.8.4, since that is what my team uses.