input: Write a python function to find the most common element in a list.
  assert most_common([1, 2, 2, 3, 3, 3, 4]) == 3
  assert most_common([10, 20, 20, 30, 30, 30, 40, 40, 40, 40]) == 40
  assert most_common(['a', 'b', 'b', 'a', 'a']) == 'a'
output: def most_common(lst):
      from collections import Counter
      count = Counter(lst)
      return count.most_common(1)[0][0]