input: Write a python function to sort a list of tuples by the second item in each tuple.
  assert sort_tuple([(1, 3), (3, 2), (2, 1)]) == [(2, 1), (3, 2), (1, 3)]
  assert sort_tuple([(1, 2), (2, 3), (3, 1)]) == [(3, 1), (1, 2), (2, 3)]
  assert sort_tuple([(5, 5), (1, 1), (4, 4)]) == [(1, 1), (4, 4), (5, 5)]
output: def sort_tuple(lst):
      return sorted(lst, key=lambda x: x[1])