old_contents: class SubjectConverter:
      regex = 'stats|calc'
  
      def to_python(self, value):
          return value
  
      def to_url(self, value):
          return value
new_contents: class SubjectConverter:
      regex = 'stats|calc'
  
      @staticmethod
      def to_python(value):
          return value
  
      @staticmethod
      def to_url(value):
          return value
  
subject: convert to static method