input: Write a python function to create a Tkinter window with a label that displays "Hello World!"
  assert create_hello_world_window() == "Tkinter window with 'Hello World!' label created"
output: def create_hello_world_window():
      import tkinter as tk
      from tkinter import ttk
      
      def main():
          root = tk.Tk()
          root.title("Hello World Window")
          label = ttk.Label(root, text="Hello World!")
          label.pack()
          root.mainloop()
          
      main()
      return "Tkinter window with 'Hello World!' label created"