input: Write a function to remove duplicates from a list.
  assert remove_duplicates([1, 2, 3, 2, 1, 4])==[1, 2, 3, 4]
  assert remove_duplicates([5, 5, 5, 5])==[5]
  assert remove_duplicates([7, 8, 9, 7, 8])==[7, 8, 9]
output: def remove_duplicates(lst):
      return list(dict.fromkeys(lst))