input: Write a python function to sort a list of tuples based on the second item of each tuple.
  assert sort_by_second_item([(1, 3), (3, 2), (2, 1)]) == [(2, 1), (3, 2), (1, 3)]
  assert sort_by_second_item([(5, 5), (1, 1), (3, 3)]) == [(1, 1), (3, 3), (5, 5)]
  assert sort_by_second_item([(10, 20), (30, 10), (20, 30)]) == [(30, 10), (10, 20), (20, 30)]
output: def sort_by_second_item(lst):
      return sorted(lst, key=lambda x: x[1])