old_contents: #!/usr/bin/env python
  # visualize-systems.py
  
  import features.visualize
  
  DIRECTORY = 'visualize-output'
  
  FORMAT = 'pdf'
  
  features.visualize.render_all(directory=DIRECTORY, format=FORMAT)
  
new_contents: #!/usr/bin/env python3
  # visualize-systems.py
  
  import features.visualize
  
  DIRECTORY = 'visualize-output'
  
  FORMAT = 'pdf'
  
  features.visualize.render_all(directory=DIRECTORY, format=FORMAT)
  
subject: use python3 for shebang