Update pool.py

Author: rekby

ydb/pool.py

@@ -5,6 +5,7 @@
 from concurrent import futures
 import collections
 import random
+import typing
 
 from . import connection as connection_impl, issues, resolver, _utilities, tracing
 from abc import abstractmethod
@@ -123,7 +124,7 @@ def subscribe(self):
             return subscription
 
     @tracing.with_trace()
-    def get(self, preferred_endpoint: Optional[EndpointKey] = None) -> Connection:
+    def get(self, preferred_endpoint: typing.Optional[EndpointKey] = None) -> Connection:
         with self.lock:
             if preferred_endpoint is not None and preferred_endpoint.node_id in self.connections_by_node_id:
                 return self.connections_by_node_id[preferred_endpoint.node_id]