PYTHON-3011 Fix test_connections_are_only_returned_once (#781)

(cherry picked from commit c404150)

Author: ShaneHarvey

test/test_load_balancer.py

@@ -45,7 +45,7 @@ def test_connections_are_only_returned_once(self):
         nconns = len(pool.sockets)
         self.db.test.find_one({})
         self.assertEqual(len(pool.sockets), nconns)
-        self.db.test.aggregate([{'$limit': 1}])
+        list(self.db.test.aggregate([{'$limit': 1}]))
         self.assertEqual(len(pool.sockets), nconns)
 
     @client_context.require_load_balancer