Python 3.8 EOL in Oct 2024

[melange396]

This has similar concerns to those in cmu-delphi/covidcast-indicators#1916 and some are also mentioned in #1019 ...

Things will not simply "stop working" when this milestone date is reached, but it may leave us open to new vulnerabilities that emerge afterward.

Python version 3.12 is (currently) the latest feature-complete version which makes it an attractive target, though migrating to that may present a number of difficulties. We can probably more easily move to 3.9 and buy ourselves another year of support and updates.