input: Write a python function to return a sorted list of unique elements.
  assert unique_list([15, 2, 4, 15, 46, 4, 3, 21]) == [2, 3, 4, 15, 21, 46]
  assert unique_list([5, 5, 5, 5, 5, 5]) == [5]
  assert unique_list([1, 2, 3, 4, 5, 6, 7, 8, 9, 10]) == [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
output: def unique_list(lst):
      return sorted(set(lst))