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