Add 3.8 and 3.9 to test to matrix, fix type bug

[pamelafox]

Purpose

Apparently some developers are using Python 3.8 to run this code, so we should run tests on 3.8 and 3.9 as well. This PR adds them to the test matrix and fixes a typing related issue.

Does this introduce a breaking change?

[ ] Yes
[X] No

Pull Request Type

What kind of change does this Pull Request introduce?

[X] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Documentation content changes
[ ] Other... Please describe:

How to Test

• Wait for tests to pass! :)