input: Write a python function to remove duplicates from a list.
  assert remove_duplicates([1, 4, 2, 6, 4, 1, 2]) == [1, 2, 4, 6]
  assert remove_duplicates([10, 20, 10, 30, 20, 30, 40]) == [10, 20, 30, 40]
  assert remove_duplicates([5, 7, 5, 9, 7, 9, 11]) == [5, 7, 9, 11]
output: def remove_duplicates(lst):
      return list(dict.fromkeys(lst))