Test setup should cover all supported Python versions

[lifeisafractal]

Currently the Tox tests are only run against Python 3.6. #37 and #36 both show Python 3.8 issues that could be cause with automated test coverage.

Proposal:

• We create a policy of what Python versions are supported. I suggest we follow upstream HomeAssistant policy of supporting the 2 most recent minor stable releases.
• Extend Tox coverage to test with both these supported versions both for local testing and for the automated testing

Immediate consequences:

• Python 3.6 would be deprecated and heading for removal in the next couple releases.

[HelloThisIsFlo]

Yes. I agree. Let's support 3.7 & 3.8 (and update tox)

For now, there's still a bug with 3.8, #37 is fixing it but I'd like to spend some time to understand how it's fixing it before merging. I'll probably have some time to look at that on Monday