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