Fix __iter__ not returning iterator (#159)

Author: henribru

rest_framework-stubs/utils/serializer_helpers.pyi

@@ -1,5 +1,5 @@
 from collections import OrderedDict
-from typing import Any, Dict, Iterable, List, MutableMapping, Tuple, Union
+from typing import Any, Dict, Iterator, List, MutableMapping, Tuple, Union
 
 from rest_framework.exceptions import ErrorDetail
 from rest_framework.fields import Field
@@ -33,7 +33,7 @@ class BoundField:
 class JSONBoundField(BoundField): ...
 
 class NestedBoundField(BoundField):
-    def __iter__(self) -> Iterable[str]: ...
+    def __iter__(self) -> Iterator[str]: ...
     def __getitem__(self, key: str) -> Union[BoundField, NestedBoundField]: ...
 
 class BindingDict(MutableMapping[str, Field]):
@@ -43,5 +43,5 @@ class BindingDict(MutableMapping[str, Field]):
     def __setitem__(self, key: str, field: Field) -> None: ...
     def __getitem__(self, key: str) -> Field: ...
     def __delitem__(self, key: str) -> None: ...
-    def __iter__(self) -> Iterable[Field]: ...  # type: ignore[override]
+    def __iter__(self) -> Iterator[Field]: ...  # type: ignore[override]
     def __len__(self) -> int: ...