PEP 705: Read-only TypedDict items

[randolf-scholz]

@Daverball

Since according to the inheritance rules outlined in PEP 705 we are allowed to override a ReadOnly key with a writeable key it might be better to call the modifier Readable to ensure future consistency, if/when we want to extend the type modifier to work with additional types.

I had a second look at this, and it seems to me now that in my comment here, it may be more appropriate to translate foo: ReadOnly[T] to __setattr__(self, name: Literal["foo"], val: Never) -> None instead of __setattr__(self, name: Literal["foo"], val: T) -> Never.

This is compatible with the intended usage here (applying to __setitem__ instead of __setattr__) and allows for contravariant overriding so that subclasses can choose to make a ReadOnly mutable.

I think if one wants to forbid writing for subtypes entirely, then Final[ReadOnly[T]] would be the correct thing to do.