input: Write a python function to return a list of numbers divisible by 3 from 1 to 20.
  assert divisible_by_3()==[3, 6, 9, 12, 15, 18]
  assert divisible_by_3()==[3, 6, 9, 12, 15, 18]
  assert divisible_by_3()==[3, 6, 9, 12, 15, 18]
output: def divisible_by_3():
      return [i for i in range(1, 21) if i % 3 == 0]