Drop support for Python 3.8

[tobiasmcnulty]

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Since adding Python 3.12 and dropping Python 3.8 support are being handled separately, I'm creating a new issue for dropping Python 3.8 support.

@bmispelon in #1574:

Some of our dependencies have started dropping support for Python 3.8 (the one we're currently running in production).

I don't think there's much to be done in this project's codebase (this is mostly an ops issue), but I wanted to have an issue where I can track which dependencies are being affected:

• [ ] docutils (+ Sphinx): #1571
• [ ] pre-commit: #1568
• [ ] watchdog: #1617

Other to dos:

• [ ] Update the pyupgrade repo in .pre-commit-config.yaml to target --py312-plus