Upgrade Python version to 3.12

[bmispelon]

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): Bump docutils from 0.17.1 to 0.21.2 in /requirements #1571
• pre-commit: Update pre-commit requirement from ~=2.20.0 to ~=3.7.1 in /requirements #1568
• watchdog: Bump watchdog from 4.0.2 to 5.0.2 in /requirements #1617

[tobiasmcnulty]

I found I couldn't upgrade these requirements without dropping Python 3.8 support, so I reverted most of them for now:

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

Assuming we want to merge Python 3.12 before dropping 3.8, perhaps we could make another ticket to drop 3.8. Happy to do so but wanted to check with everyone beforehand on how we want to split it up.