Add rrule.__iter__ return type (#9828)

Author: denyszhak

stubs/python-dateutil/dateutil/rrule.pyi

@@ -1,6 +1,6 @@
 import datetime
 from _typeshed import Incomplete
-from collections.abc import Iterable
+from collections.abc import Iterable, Iterator
 from typing import Any
 from typing_extensions import TypeAlias
 
@@ -27,7 +27,7 @@ SU: weekday
 
 class rrulebase:
     def __init__(self, cache: bool = ...) -> None: ...
-    def __iter__(self): ...
+    def __iter__(self) -> Iterator[datetime.datetime]: ...
     def __getitem__(self, item): ...
     def __contains__(self, item): ...
     def count(self): ...