Add Python 3.9 Support

[sco1]

With Python 3.9 entering beta we're at a good point to begin to evaluate for any potential issues with the new version. This should be an uneventful update; local runs of the test suite with 3.9.0b1 presents no issues.

This is stalled pending CI support for Python 3.9. See: actions/runner-images#912

Python 3.9 final release is expected in the October timeframe.

[sco1]

Per: actions/setup-python#20 (comment)

The setup-python action should be capable of handling pre-release Python versions now, so this can be unstalled.

edit: Maybe now would be as good a time as any to look into switching over to GH Actions.

[sco1]

Will supersede this with a more generic CI issue. There are currently no issues with the test suite when running 3.9 locally.