input: Write a python function to shuffle a list of letters and update button texts in a Tkinter window.
  assert set(shuffle()) == set('ABCDEFGHIJKLMNOPQRSTUVWXYZ')
  assert len(shuffle()) == 25
output: import random
  import tkinter as tk
  
  def shuffle():
      letters = list('ABCDEFGHIJKLMNOPQRSTUVWXYZ')
      random.shuffle(letters)
      return letters[:25]
  
  def update_buttons():
      shuffled_letters = shuffle()
      for i, button in enumerate(buttons):
          button.config(text=shuffled_letters[i])
  
  def create_window():
      global buttons
      window = tk.Tk()
      buttons = []
      for i in range(25):
          btn = tk.Button(window, text='', width=4, height=2)
          btn.grid(row=i//5, column=i%5)
          buttons.append(btn)
      
      update_button = tk.Button(window, text='Shuffle', command=update_buttons)
      update_button.grid(row=5, column=0, columnspan=5)
      
      window.mainloop()
  
  create_window()