Python 3.9 support

[mLewisLogic]

Maybe it already works?
Adding this as an issue to remind us to enable it in CI and see how it goes.

[cjdsellers]

I noticed the PyPI badge added 3.9 a few days ago, almost as if we added the classifier for it.

I would expect everything is backwards compatible so I'll add it to the python-version list and see how it goes.

[cjdsellers]

Looks like it works 7faf02e

[cjdsellers]

Closing as the CI build is working with Python 3.9. We can reopen if a specific Python 3.9 issue comes up.