Compatibility with newer MyPy versions

The newer versions always have a message, even on success.

tests/test_mypy.py

@@ -23,5 +23,5 @@ def test_run_mypy(self):
             messages.append(stdout)
         if status:
             messages.append('Mypy failed with status %d' % status)
-        if messages:
+        if messages and not all('Success' in message for message in messages):
             self.fail('\n'.join(['Mypy errors:'] + messages))