old_contents: class Tabpage(object):
      pass
  
new_contents: from util import RemoteMap
  
  class Tabpage(object):
      @property
      def vars(self):
          if not hasattr(self, '_vars'):
              self._vars = RemoteMap(lambda k: self.get_var(k),
                                     lambda k, v: self.set_var(k, v))
          return self._vars
  
      @property
      def number(self):
          return self._handle
  
      @property
      def window(self):
          return self.get_window()
  
      @property
      def valid(self):
          return self.is_valid()
  
subject: Add tabpage mixin methods