Use more specialized type hinting from Python `typing` library.

[LswaN58]

There are several type hints I’ve read about that we should start using, given in checklist below.

• [x] Self : For any functions that return instance of given class, use typing.Self as the return type e.g. def FromJSON(...) -> Self: Edit: on further review, does not work this way...
• [x] Final : For any consts, use Final on the type, e.g. self.MAX_VAL : Final[int] = 50
• [x] (optional) TypeGuard : Not sure if we use this or not, but if a function is used to check that a variable is the right type, and returns true if so, we can do def foo(bar) -> TypeGuard[int] and then type-checker knows that if foo(baz) == True, then baz is of type int. Might maybe be of use in schemas, but this seems like an outside chance.

[LswaN58]

Finished in c75b843a2fba8099f1570ba5592cb37bea7cf60e, minus a few features that don't use Final[...] yet.