ci: remove python 3.8 support

[terriko]

Python 3.8 is no longer getting security updates, so we'll be dropping support. It's likely that things will continue to work for some time, but we'll stop running tests and will be removing linter workarounds and pyupgrade stuff.

Todo list:

• remove python 3.8 from testing.yml
• remove any package workarounds that reference python 3.8 (there's some in dev-requirements.txt for sure)
• change configuration for pyupgrade in .pre-commit-config.yaml
• look through code to see if we have other python 3.8 specific workarounds that should be flagged for eventual removal (can set up separate issues for these if we don't want to pull them out right away). There might be a couple in the tests specifically.

This should be pretty easy to do, the only reason I'm not flagging it as a good first issue is that I'm going to be swamped this week so I won't have time to walk someone through it beyond what's written here. Anyone should feel welcome to pick this up OR speak now on this issue if you absolutely need us to keep 3.8 support a bit longer.