input: Write a python function to sort a list of tuples based on the second element.
  assert sort_tuples([(4, 20), (3, -5), (10, 0)]) == [(3, -5), (10, 0), (4, 20)]
  assert sort_tuples([(1, 3), (3, 1), (2, 2)]) == [(3, 1), (2, 2), (1, 3)]
  assert sort_tuples([(5, 10), (1, 2), (4, 9)]) == [(1, 2), (4, 9), (5, 10)]
output: def sort_tuples(lst):
      return sorted(lst, key=lambda x: x[1])