docs: def _download(self):
          """Download NApp package from server.
  
          Return:
              str: Downloaded temp filename.
  
          Raises:
              urllib.error.HTTPError: If download is not successful.
  
          """
          repo = self._config.get('napps', 'repo')
          napp_id = '{}/{}-{}.napp'.format(self.user, self.napp, self.version)
          uri = os.path.join(repo, napp_id)
          return urllib.request.urlretrieve(uri)[0]