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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."""

libagent/device/trezor_defs.py

@@ -28,3 +28,4 @@ def find_device():
         return get_transport(os.environ.get("TREZOR_PATH"))
     except Exception as e:  # pylint: disable=broad-except
         log.debug("Failed to find a Trezor device: %s", e)
+        return None