input: Write a python function to generate a list of the first 20 perfect squares.
  assert generate_squares() == [1, 4, 9, 16, 25, 36, 49, 64, 81, 100, 121, 144, 169, 196, 225, 256, 289, 324, 361, 400]
output: def generate_squares():
      return [i ** 2 for i in range(1, 21)]