Extract the part of the code described below:
    Here is a simple tic-tac-toe game that you can use as a starting point:
```
import pygame

Initialize pygame

pygame.init()

Set up the window

WINDOWSIZE = (500, 500)
screen = pygame.display.setmode(WINDOW_SIZE)

Define some colors

BLACK = (0, 0, 0)
WHITE = (255, 255, 255)
GRAY = (128, 128, 128)
```
