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.

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]:
     """

pandas/core/series.py

@@ -1926,7 +1926,7 @@ def keys(self) -> Index:
         """
         return self.index
 
-    def to_dict(self, into: type[dict] = dict) -> dict:
+    def to_dict(self, into: type[dict] | dict = dict) -> dict:
         """
         Convert Series to {label -> value} dict or dict-like object.