Use `typing.Self` consistently

[disinvite]

When you need to reference the type of a class inside of the class declaration, your options are to quote the class name:

class Hello:
    @classmethod
    def create(cls) -> "Hello":
        pass

or use typing.Self, introduced in Python 3.11.

We still support Python 3.10 so we need to import Self from typing_extensions instead.