Correct type annotation for DataFrame.to_dict.

The `into` argument of DataFrame.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.

pandas/core/frame.py

@@ -1925,7 +1925,7 @@ def _create_data_for_split_and_tight_to_dict(
     def to_dict(
         self,
         orient: Literal["dict", "list", "series", "split", "tight", "index"] = ...,
-        into: type[dict] = ...,
+        into: type[dict] | dict = ...,
         index: bool = ...,
     ) -> dict:
         ...
@@ -1934,7 +1934,7 @@ def to_dict(
     def to_dict(
         self,
         orient: Literal["records"],
-        into: type[dict] = ...,
+        into: type[dict] | dict = ...,
         index: bool = ...,
     ) -> list[dict]:
         ...
@@ -1947,7 +1947,7 @@ def to_dict(
         orient: Literal[
             "dict", "list", "series", "split", "tight", "records", "index"
         ] = "dict",
-        into: type[dict] = dict,
+        into: type[dict] | dict = dict,
         index: bool = True,
     ) -> dict | list[dict]:
         """

pandas/core/methods/to_dict.py

@@ -24,7 +24,7 @@ def to_dict(
     orient: Literal[
         "dict", "list", "series", "split", "tight", "records", "index"
     ] = "dict",
-    into: type[dict] = dict,
+    into: type[dict] | dict = dict,
     index: bool = True,
 ) -> dict | list[dict]:
     """