stubtest: basic support for unpack kwargs

[hauntsaninja]

Fixes #21023

[brianschubert]

I'd suggest adding some cases for mismatched optional parameters (e.g. optional in the TypedDict but required at runtime, and vice versa)

[brianschubert]

Testing this locally, I think you need to set kind to ARG_NAMED_OPT too for optional parameters to be handled correctly.

Suggested change

	stub_sig.kwonly[key_name] = nodes.Argument(
	nodes.Var(key_name, key_type),
	type_annotation=key_type,
	initializer=(
	nodes.EllipsisExpr()
	if key_name not in typed_dict_arg.required_keys
	else None
	),
	kind=nodes.ARG_NAMED,
	pos_only=False,
	optional = key_name not in typed_dict_arg.required_keys
	stub_sig.kwonly[key_name] = nodes.Argument(
	nodes.Var(key_name, key_type),
	type_annotation=key_type,
	initializer=nodes.EllipsisExpr() if optional else None,
	kind=nodes.ARG_NAMED_OPT if optional else nodes.ARG_NAMED,
	pos_only=False,

[brianschubert]

(I'm also not sure if setting initializer=nodes.EllipsisExpr() is necessary. I wasn't able to find a difference between using EllipsisExpr vs always using None)

[brianschubert]

So that I won't have to update #19574 again :-)

Suggested change

	from typing import TypedDict, Unpack
	class _Args(TypedDict):
	from typing import TypedDict, Unpack, type_check_only
	@type_check_only
	class _Args(TypedDict):