Conformance: test that `@override` checks are honored for `__init__` / `__new__`

[randolf-scholz]

When explicitly marking a method with @override, type checkers ought to test assignability (https://typing.python.org/en/latest/spec/class-compat.html#override).

Hence, even methods like __init__ and __new__, which are normally not checked against their parent classes, should be checked when explicilty decorated with @override.

from typing import override

class Foo:
    def __init__(self, x: int) -> None: ...

class Bar(Foo):
    @override
    def __init__(self, y: int) -> None: ...  # should error

This is useful in cases when we really need constructors to be compatible.
I suggest adding a version of the example above to the conformance test suite.

[JelleZijlstra]

That is indeed what the spec says, though according to the linked ty issue no type checker has implemented it. I think it's reasonable to interpret it this way, but it would be nice if some type checkers implemented this rule. If we agree this is the right direction though, type checker implementation doesn't need to block adding this to the conformance tests.

[randolf-scholz]

pyrefly now implements this behavior.