Add support for Python 3.9

[micahellison]

Feature Request

It will soon be time to officially support Python 3.9. The final beta came out recently and the release candidate phase begins this month. It would be nice to support the official release in a timely manner, especially since we've just deprecated 3.6.

This change should involve:

• documentation
• Travis CI builds
• poetry
• Perhaps some other things (it would be worth searching the code base for "3.8")

[wren]

We should probably add 3.9 tests to the suite sooner, and mark them as allow_failures.