Python 3.8 was released yesterday (October 14, 2019), but we don't officially support it yet.

So, let's:

• Add 3.8-dev to our TravisCI test matrix so that tests run against Python 3.8 as well.
• Update setup.py to add the 3.8 trove classifier.

If any tests fail, we'll need to fix them (in separate PRs) beforehand.