Misc. Django/Python improvements

[mvandenburgh]

I noticed a bunch of code that did things in a slightly incorrect and/or outdated way, this PR fixes them -

1. Updates any use of .filter(~Q(...)) to .exclude(...). exclude behaves the same and is the "correct" way to do this sort of query in Django
2. Update DRF viewsets to use rest_framework.generics.get_object_or_404 instead of django.shortcuts.get_object_or_404.
   • DRF's version of the function wraps the vanilla Django one and adds some DRF specific validation/error handling https://stackoverflow.com/questions/68001954/difference-between-django-shortcuts-and-rest-framework-generics
3. Replace instances of typing.Union and typing.Optional with new union type syntax

Technically, 3) isn't necessarily incorrect (from what I understand, typing.Optional and typing.Union are not deprecated, at least not yet), but I find the x | y union syntax to be much more readable. I don't feel strongly on this though 🤷‍♂️

[jjnesbitt]

LGTM

[dandibot]

🚀 PR was released in v0.2.18 🚀