Adding a stub for redis.Redis.client() (#4720)

Closes: #4721

third_party/2and3/redis/client.pyi

@@ -369,6 +369,7 @@ class Redis(object):
     def cluster(self, cluster_arg: str, *args: Any) -> Any: ...
     def __enter__(self) -> Redis: ...
     def __exit__(self, exc_type, exc_value, traceback): ...
+    def client(self) -> Redis: ...
 
 StrictRedis = Redis