Add test steps and explicit support for Python 3.9

[tsalo]

Summary

We currently only explicitly support Python versions 3.6, 3.7, and 3.8. I'd like to extend that support to 3.9.

Next steps

1. Add 3.9 to the classifiers within info.py.
2. Create new job for 3.9 in the CircleCI config file. You can just copy the job for 3.8 and change the relevant variables/names.
3. Add the new job to the CircleCI workflow section in the config file.

[tsalo]

Python 3.10 will have to wait until all of our dependencies (esp. numpy) have working 3.10 versions.