Sunset support for Python 3.9

[dandye]

Python 3.9 is now beyond end-of-life, so we can remove support for it [1]

With this deprecation, we can start using the enhancements from v3.10 [2]

[1] https://devguide.python.org/versions/
[2] https://docs.python.org/3/whatsnew/3.10.html

[mihirvala08]

Addressed in #156