PEP 821: Support for unpacking TypedDicts in Callable type hints

This PEP proposes allowing Unpack[TypedDict] in the parameter list inside Callable, enabling concise and type-safe ways to describe keyword-only callable signatures. Currently, Callable assumes positional-only parameters, and typing keyword-only functions requires verbose callback protocols. With this proposal, the keyword structure defined by a TypedDict can be reused directly in Callable.