Serializer iterator returns BoundField, not str. (#161)

Author: danielroseman

rest_framework-stubs/serializers.pyi

@@ -152,7 +152,7 @@ class Serializer(
     def fields(self) -> BindingDict: ...
     def get_fields(self) -> Dict[str, Field]: ...
     def validate(self, attrs: Any) -> Any: ...
-    def __iter__(self) -> Iterator[str]: ...
+    def __iter__(self) -> Iterator[BoundField]: ...
     def __getitem__(self, key: str) -> BoundField: ...
     def _read_only_defaults(self) -> Dict[str, Any]: ...
     @property