input: Write a python function to remove duplicates from a list.
  assert unique_list([1,2,2,3,3,3]) == [1, 2, 3]
  assert unique_list([4,4,4,4,5,5,6,6,6]) == [4, 5, 6]
  assert unique_list([7,8,9,9,8,7]) == [7, 8, 9]
output: def unique_list(lst):
      return list(set(lst))