old_contents: def hamming(s1,s2):
      return sum(1 for a,b in map(None,s1,s2) if a != b)
  
new_contents: import sys
  
  if sys.version_info[0] == 2:
      from itertools import izip_longest as zip_longest
  else:
      from itertools import zip_longest
  
  
  def hamming(s1, s2):
      return sum(1 for a, b in zip_longest(s1, s2) if a != b)
  
subject: Make hamming exercise compatible