Converts an image to grayscale using the crude average of the r, g, and b

def gray_scale(img):
  img_height = len(img)
  img_width = len(img[0])
  gray_img = []
  for i in range(img_height):
    gray_img.append([])
    for j in range(img_width):
      gray_img[i].append(0)
      for k in range(3):
        gray_img[i][j] = gray_img[i][j] + img[i][j][k]
      gray_img[i][j] = gray_img[i][j]/3
  return gray_img