Compatibility with newer MyPy versions

The newer versions always have a message, even on success.
diff --git a/tests/test_mypy.py b/tests/test_mypy.py
index c2a9745..8258e7e 100644
--- a/tests/test_mypy.py
+++ b/tests/test_mypy.py
@@ -23,5 +23,5 @@
             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))