old_contents: class Matrix(object):
      def __init__(self, s):
          self.rows = [[int(n) for n in row.split()]
                       for row in s.split('\n')]
  
      @property
      def columns(self):
          return map(list, zip(*self.rows))
  
new_contents: class Matrix(object):
      def __init__(self, s):
          self.rows = [[int(n) for n in row.split()]
                       for row in s.split('\n')]
  
      @property
      def columns(self):
          return [list(tup) for tup in zip(*self.rows)]
  
subject: Make matrix exercise compatible with Python3