docs: def next_formatted_pair(self):
        """\
        Returns a :class:`FormattedPair <shorten.store.FormattedPair>` containing 
        attributes `key`, `token`, `formatted_key` and `formatted_token`. 
        Calling this method will always consume a key and token.
        """
  
        key = self.key_gen.next()
        token = self.token_gen.create_token(key)
        fkey = self.formatter.format_key(key)
        ftoken = self.formatter.format_token(token)
  
        return FormattedPair(key, token, fkey, ftoken)