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(['a', 'b', 'b', 'c', 'c', 'c']) == 'c'
  assert most_common([1, 1, 2, 2, 2, 3, 3]) == 2
output: def most_common(lst):
      from collections import Counter
      counter = Counter(lst)
      return counter.most_common(1)[0][0]