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Fixup a few pylint comments

libagent/device/trezor.py

@@ -71,6 +71,7 @@ class Trezor(interface.Device):
                 log.exception('ping failed: %s', e)
                 connection.close()  # so the next HID open() will succeed
                 raise
+        return None
 
     def close(self):
         """Close connection."""