conformance: make open TypedDict extra kwargs optional

[charliermarsh]

Summary

#2272 made errors optional when an extra keyword is passed to **kwargs: Unpack[OpenTypedDict], modifying the expectation in callables_kwargs.py. But typeddicts_extra_items.py contains the same case:

class MovieNoExtra(TypedDict):
    name: str

def unpack_no_extra(**kwargs: Unpack[MovieNoExtra]) ->
None: ...

unpack_no_extra(name="No Country for Old Men",
year=2007)  # E?

This PR marks the duplicate case optional, for consistency with #2272.

[JelleZijlstra]

Agree this seems exactly analogous to the change in #2272.