conformance: allow consistent treatment of kwargs: Unpack[TD]

[carljm]

The conformance suite currently asserts this (simplified for clarity):

class TD(TypedDict):
	v1: int

def f(**kwargs: Unpack[TD]): ...

f(v1=1, v2=2) # E: TD doesn't contain `v2`

This implies that the signature of f de-sugars to f(*, v1: int). But the current conformance suite simultaneously also requires that callable assignability behave as if it de-sugars to f(*, v1: int, **kwargs: object). That addition to the conformance suite was justified by this call, which is allowed by pyright and pyrefly:

class TD2(TD1):
    v2: int

def _(td2: TD2):
    f(**td2)

It seems reasonable to allow this call, because TD2 is a subtype of TD1 and assignable to it, so **td2 should therefore be acceptable for **kwargs: Unpack[TD1]. But if this call (which definitely provides a keyword argument v2) is allowed, then we should also allow the above call that explicitly provides v2.

I think we could defensibly pick either of the interpretations of the signature of f (with or without **kwargs: object), but type checkers should be allowed to pick a consistent interpretation, not be required to implement an inconsistent hybrid behavior.

The interpretation with **kwargs: object is IMO more consistent and better supported by the spec. The spec says that if TD has extra_items, then arbitrary additional keyword arguments of that type should be accepted. And it also says that a TypedDict without an explicit closed or extra_items parameter usually behaves as if it has extra_items of type ReadOnly[object].

The interpretation without **kwargs: object would effectively be saying that **kwargs: Unpack[TD] is one of these special cases (along with TypedDict constructor calls) where an unmarked TypedDict is (inconsistent with its "open" semantics) not treated as having extra_items of type ReadOnly[object].

We can have a discussion about which of these interpretations should be preferred, but given the lack of clarity about this in the spec, I think for now a conformant implementation should be allowed to choose either interpretation and implement it consistently.