input: Write a function to remove duplicates from a list.
  assert remove_duplicates([1,1,2,3,4,4,5,6])==[1, 2, 3, 4, 5, 6]
  assert remove_duplicates([10,20,20,30,40,40,50,50])==[10, 20, 30, 40, 50]
  assert remove_duplicates([5,4,3,2,1,1,2,3,4,5])==[5, 4, 3, 2, 1]
output: def remove_duplicates(lst):
      return list(dict.fromkeys(lst))