old_contents: def increment_last_byte(byte_string):
      return byte_string[:-1] + bytes([byte_string[-1] + 1])
  
new_contents: def increment_last_byte(byte_string):
      s = bytearray(byte_string)
      s[-1] = s[-1] + 1
      return bytes(s)
  
subject: Fix increment_last_byte on py27