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