Support for Python 3.9

[kclowes]

• Version: 5.x
• Python: 3.9!
• OS: osx/linux/win

What was wrong?

Now that Python 3.9 is out, we should make sure we can support it.

How can it be fixed?

Pretty much anywhere that tests against Python 3.7 and 3.8, we should add Python 3.9. Python 3.8 support was added #1471, so that would be a good template to follow.

[wolovim]

closed via #1774