Upgrade to Python 3.12

[milescalabresi]

Depends on #3524. Minor versions of Python should be backwards compatible, but it's important to check. This issue includes checking package dependencies (and upgrading any that block this issue), upgrading all sites, requirement files, the dev server setup (and associated instructions), any scripts that call out the version explicitly, and anywhere else you can think of.

[hwatheod]

Relevant docs:

https://docs.python.org/3/whatsnew/3.8.html#porting-to-python-3-8
https://docs.python.org/3/whatsnew/3.9.html#porting-to-python-3-9
https://docs.python.org/3/whatsnew/3.10.html#porting-to-python-3-10
https://docs.python.org/3/whatsnew/3.11.html#porting-to-python-3-11
https://docs.python.org/3/whatsnew/3.12.html#porting-to-python-3-12