Add return type for dateutil.rrule._rrulestr.__call__. (#7868)

stubs/python-dateutil/dateutil/rrule.pyi

@@ -105,6 +105,6 @@ class rruleset(rrulebase):
     def exdate(self, exdate): ...
 
 class _rrulestr:
-    def __call__(self, s, **kwargs): ...
+    def __call__(self, s, **kwargs) -> rrule | rruleset: ...
 
 rrulestr: _rrulestr