Upgrade Python version

[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