Drop support for Python 3.9

[joshuadavidthomas]

It's coming up on EOL very shortly, and once #86 is done none of the supported Django versions depend on it.

[joshuadavidthomas]

Ah nevermind, 4.2 supports Python 3.9. Keep it around for now until EOL in October: https://devguide.python.org/versions/#supported-versions