input: Write a python function to find the second most frequent element in a list.
  assert second_most_frequent([1, 2, 3, 3, 3, 2, 4, 4, 1, 5]) == 2
  assert second_most_frequent(['a', 'b', 'b', 'c', 'c', 'c', 'a', 'a', 'd', 'd', 'd', 'd']) == 'c'
  assert second_most_frequent([5, 5, 5, 6, 6, 7, 7, 7, 7, 8, 8, 8, 8, 8]) == 7
output: def second_most_frequent(lst):
      from collections import Counter
      frequency = Counter(lst)
      most_common = frequency.most_common()
      return most_common[1][0] if len(most_common) > 1 else None