input: Write a python function to convert a list of RGB tuples to their hexadecimal color codes.
  assert rgb_to_hex([(255, 0, 0), (0, 255, 0), (0, 0, 255)]) == ["#ff0000", "#00ff00", "#0000ff"]
  assert rgb_to_hex([(255, 255, 255), (0, 0, 0)]) == ["#ffffff", "#000000"]
  assert rgb_to_hex([(128, 128, 128), (192, 192, 192), (64, 64, 64)]) == ["#808080", "#c0c0c0", "#404040"]
output: def rgb_to_hex(rgb_list):
      return ['#{:02x}{:02x}{:02x}'.format(r, g, b) for r, g, b in rgb_list]