Evalml: Support python 3.8

[dsherry]

Python 3.8 was released in Oct 2019.

In order to support 3.8, we should set up a new copy of the CircleCI tests to run on 3.8. We can use the first results to determine what if any dev work is needed. I don't see any major backwards-incompatible changes listed, so hopefully it'll be easy.

(Related: #167)

[dsherry]

Hmm, I just realized, I'm pretty sure I've been running evalml locally on 3.8 for a couple months with no issues. So that's a good sign :)

[dsherry]

@angela97lin and I ran into a weird error today. Something about the flake8 version I'm running locally with python 3.8.1 causes some Imports are incorrectly sorted messages on which don't show up on python 3.7.9. We noticed them now because they seem to be specific to her new catboost changes from #247 . So when we tackle this issue, we'll probably need to put up a PR to fix those.