input: Write a function to sort a list of tuples by the second item in each tuple.
  assert sort_by_second_item([(1, 3), (3, 2), (5, 1)]) == [(5, 1), (3, 2), (1, 3)]
  assert sort_by_second_item([(4, 4), (3, 3), (2, 2), (1, 1)]) == [(1, 1), (2, 2), (3, 3), (4, 4)]
  assert sort_by_second_item([(2, 3), (1, 2), (3, 1), (4, 0)]) == [(4, 0), (3, 1), (1, 2), (2, 3)]
output: def sort_by_second_item(lst):
      return sorted(lst, key=lambda x: x[1])