TYP: Correct type annotation for to_dict.
The `into` argument of DataFrame.to_dict and Series.to_dict can be either a class or instance of a class of dict; this is covariant - subclasses of dict can also be used. The argument was annotated as `type[dict]` though, so type checkers marked passing initialized objects (required for collections.defaultdict) as an incorrect argument type.

Fix by annotating `into` to take either a subclass of dict or an initialized instance of a subclass of dict.
Thanks @jsspencer 