old_contents: #! /usr/bin/env python
  
  try:
      # python 2
      import Tkinter as tk
      from tkMessageBox import Message
  except ImportError:
      # python 3
      import tkinter as tk
      from tkinter.messagebox import Message
  import lib
  
  if __name__ == '__main__':
      main_window = lib.gui.MainWindow(16,13)
      main_window.mainloop()
  
new_contents: #! /usr/bin/env python
  
  import lib
  
  if __name__ == '__main__':
      main_window = lib.gui.MainWindow(16,13)
      main_window.mainloop()
  
subject: remove Tkinter import (not needed there any more)