python3: modify exception string (#40012)

tests/test_cook.py

@@ -178,9 +178,8 @@ def test_wait_operationals(db, monkeypatch):
 
     # not operational
     obj2.last_operational_success_timestamp = None
-    with pytest.raises(CommandError) as e_info:
+    with pytest.raises(CommandError, match='timeout waiting for url2'):
         command.wait_operationals(.6)
-    assert str(e_info).find('CommandError: timeout waiting for url2') != -1
 
 def test_set_variable(db):
     command = Command()