input: Write a function to remove duplicate elements from a list.
  assert remove_duplicates([1,2,3,3,2,1,4,5])==[1,2,3,4,5]
  assert remove_duplicates([10,20,10,30,40,30,20])==[10,20,30,40]
  assert remove_duplicates([5,7,9,8,6,4,3,5,7,9])==[5,7,9,8,6,4,3]
output: def remove_duplicates(lst):
      return list(dict.fromkeys(lst))