`called` is just an attribute, so this isn't actually checking that it was called.  The exception is:
```
exceptions.AssertionError: Expected call: mock()
Actual call: mock(<buildbot.config.MasterConfig object at 0x1fa6b90>)
```
so the method is being called with an argument, but you're asserting that it is called with no arguments.  The `mock.ANY` argument can help here:

```
self.master.reconfigService.assert_called_with(mock.ANY)
```

with that change, this commit can be merged.