input: Write a python function to merge two lists without duplicates.
  assert merge_lists([1, 2, 3], [3, 4, 5]) == [1, 2, 3, 4, 5]
  assert merge_lists([10, 20, 30], [30, 40, 50]) == [10, 20, 30, 40, 50]
  assert merge_lists([5, 7, 9], [7, 8, 9, 10]) == [5, 7, 9, 8, 10]
output: def merge_lists(list1, list2):
      return list(dict.fromkeys(list1 + list2))