Fix deque mutated while iterating error

Author: technige

neo4j/v1/session.py

@@ -140,7 +140,7 @@ def recycle(self, session):
         :return:
         """
         pool = self.session_pool
-        for s in pool:
+        for s in list(pool):  # freezing the pool into a list for iteration allows pool mutation inside the loop
             if not s.healthy:
                 pool.remove(s)
         if session.healthy and len(pool) < self.max_pool_size and session not in pool: