ReadOnly TypedDict items should be treated as covariant

[JukkaL]

ReadOnly was implemented in #17644. I think the implementation is incomplete -- this generates a false positive:

from typing import TypedDict
from typing_extensions import ReadOnly

class A(TypedDict):
    name: ReadOnly[str | None]

class B(TypedDict):
    name: str

def f(a: A) -> None: ...

b: B = {"name": "x"}
f(b)  # Incompatible type "B"; expected "A"

Extract from PEP 705:

Read-only items behave covariantly, as they cannot be mutated.

cc @sobolevn

[sobolevn]

Yes, I agree. I will fix this before 1.12

[JukkaL]

@sobolevn I created a fix since it was pretty simple (#17904).