input: Write a function to find the most common element in a list.
  assert most_common([1, 2, 2, 3, 4, 4, 4, 5]) == 4
  assert most_common(['a', 'b', 'b', 'c', 'a', 'a', 'a']) == 'a'
  assert most_common([1, 1, 2, 2, 3, 3, 3, 2, 2]) == 2
output: def most_common(lst):
      from collections import Counter
      return Counter(lst).most_common(1)[0][0]