input: Write a python function to sort a list of tuples by the second element.
  assert sort_tuples_by_second_element([(1, 3), (4, 2), (2, 1), (3, 4)]) == [(2, 1), (4, 2), (1, 3), (3, 4)]
  assert sort_tuples_by_second_element([(10, 5), (3, 7), (4, 2), (2, 9)]) == [(4, 2), (10, 5), (3, 7), (2, 9)]
  assert sort_tuples_by_second_element([(5, 10), (1, 5), (7, 3), (9, 1)]) == [(9, 1), (7, 3), (1, 5), (5, 10)]
output: def sort_tuples_by_second_element(lst):
      return sorted(lst, key=lambda x: x[1])