Small fix for modern flake8. (#539)

It wants an additional space.

Signed-off-by: Chris Lalancette <clalancette@gmail.com>

test_communication/test/subscriber_py.py

@@ -68,7 +68,7 @@ def listener(message_name, namespace):
     node.destroy_node()
     rclpy.shutdown()
 
-    assert len(received_messages) == len(expected_msgs),\
+    assert len(received_messages) == len(expected_msgs), \
         'Should have received {} {} messages from talker'.format(len(expected_msgs), message_name)