refactor: use `Self` instead of class name

[mvadari]

High Level Overview of Change

This PR replaces the self: ClassName paradigm with self: Self, added in Python 3.11 but supported in typing_extensions for all versions. This allows us to remove most of the awkward, unintuitive uses of TypeVar.

Context of Change

It's cleaner and better for handling inheritance.

Type of Change

• [x] Refactor (non-breaking change that only restructures code)

Did you update CHANGELOG.md?

• [x] No, this change does not impact library users

Test Plan

CI passes.

[mvadari]

That was a lot to cover but LGTM 😄

It's all just the same change over and over again 😅