input: Write a python function to find the most frequent element in a list.
  assert most_frequent([2, 2, 2, 3, 3, 3, 4, 6, 7, 6]) == 2
  assert most_frequent([1, 1, 2, 3, 4, 4, 4, 5, 5, 5, 5]) == 5
  assert most_frequent([1, 2, 3, 4, 4, 5, 6, 7, 8, 8, 8, 8]) == 8
  assert most_frequent([]) == None
output: def most_frequent(lst):
      if not lst:
          return None
      return max(set(lst), key=lst.count)