title: Tkinter Checkbutton not updating when changing variable
body: <p>I have a tkinter GUI that I'm working on in Python 3.8 on my Macbook. I've encountered a problem where changing the variable associated with a checkbutton doesn't change the appearance of the checkbutton itself. I'd like the checkbutton to show up as checked if I set the IntVar() associated with it to 1, and from everything I've read, this should be happening.</p>
  
  <p>Here's some extremely simplified code showing the problem:</p>
  
  <pre><code>import tkinter as tk
  
  class Window():
      def __init__(self, master):
          var = tk.IntVar()
          checkbutton = tk.Checkbutton(master, variable=var)
          checkbutton.pack()
          var.set(1)
  
  root = tk.Tk()
  Window(root)
  root.mainloop()
  </code></pre>
  
  <p>When I run the script, the checkbutton isn't checked. I am still able to check the checkbutton by clicking on it though. Is this a known bug or am I missing something?</p>