Use new features from Python 3.12 throughout API code

[taesungh]

Following from #388/#565, the serverless function for the API is now deployed with Python 3.12. Accordingly, there are a few new language features we could be using along with other things to update. These should ideally happen in phases rather than all at once.

General

• Update checks workflow and Dockerfile to use Python 3.12
• Check if overriding AsyncIOMotorClient.get_io_loop (mongodb_handler.py:23) is still necessary

New in Python 3.10

• Use new type union operator (e.g. X | Y) instead of typing.Union
• Import TypeAlias from typing rather than typing_extensions

New in Python 3.11

• Use the native enum.StrEnum
  • Be careful to check there are no regressions (e.g. might need .value in some places or no longer need in others)
• Import assert_never from typing rather than typing_extensions

New in Python 3.12

• Replace TypeAlias with new type statements
• Use the new itertools.batched rather than our own copy in utils.batched