Fixing the return type hint for the transaction method in the standalone client. (#3660)

* Fixing the return type hint for transaction method in standalone client

* Changing Optional to explicitly adding None with Union - to improve clarity

Author: petyaslavova

redis/client.py

@@ -450,7 +450,7 @@ def pipeline(self, transaction=True, shard_hint=None) -> "Pipeline":
 
     def transaction(
         self, func: Callable[["Pipeline"], None], *watches, **kwargs
-    ) -> None:
+    ) -> Union[List[Any], Any, None]:
         """
         Convenience method for executing the callable `func` as a transaction
         while watching all keys specified in `watches`. The 'func' callable