Add a go/pytype annotation on Duration.toTimedelta.

PiperOrigin-RevId: 616938980

python/google/protobuf/internal/well_known_types.py

@@ -400,7 +400,7 @@ class Duration(object):
     self.seconds = seconds
     self.nanos = 0
 
-  def ToTimedelta(self):
+  def ToTimedelta(self) -> datetime.timedelta:
     """Converts Duration to timedelta."""
     return datetime.timedelta(
         seconds=self.seconds, microseconds=_RoundTowardZero(