Resolve Error: "AttributeError"

fix: tk11_msgbox.py

Add a line to resolve "AttributeError" : 
import tkinter.messagebox

Author: JNingWei

tkinterTUT/tk11_msgbox.py

@@ -4,6 +4,7 @@
 # Youku video tutorial: http://i.youku.com/pythontutorial
 
 import tkinter as tk
+import tkinter.messagebox
 
 window = tk.Tk()
 window.title('my window')