[ty] Extend Final test suite

[sharkdp]

Summary

Restructures and cleans up the typing.Final test suite. Also adds a few more tests with TODOs based on the typing spec for typing.Final.

[AlexWaygood]

how does __new__ play into things here? (I can't remember what the spec says.) E.g. this feels ~fine to me... but if the spec disallows it, then the spec disallows it, I guess:

from typing import Final

class Foo:
    def __new__(cls):
        self = object.__new__(cls)
        self.x: Final = 42
        return self

it's basically the pattern that fractions.Fraction uses in the stdlib: https://github.com/python/cpython/blob/9a21df7c0a494e2819775eabd522ebec994d96c0/Lib/fractions.py#L205-L311

[sharkdp]

The spec says (emphasis mine):

Finally, as self.id: Final = 1 (also optionally with a type in square brackets). This is allowed only in __init__ methods, so that the final instance attribute is assigned only once when an instance is created.

In your code snippet in particular, any declaration would be illegal in that position. You can't annotate attribute assignments if they don't refer to the first parameter in a method (ty does not yet enforce this, see astral-sh/ty#509).

[AlexWaygood]

hmm, I think there might be value in special-casing __new__ methods specifically so that we allow attribute annotations in __new__ methods where the value of the attribute expression is an instance of cls. But we can defer that discussion to a later PR