code: def _grab_crl(user_agent, url, timeout):
      
      request = Request(url)
      request.add_header('Accept', 'application/pkix-crl')
      request.add_header('User-Agent', user_agent)
      response = urlopen(request, None, timeout)
      data = response.read()
      if pem.detect(data):
          _, _, data = pem.unarmor(data)
      return crl.CertificateList.load(data)
docs: Fetches a CRL and parses it
  
      :param user_agent:
          A unicode string of the user agent to use when fetching the URL
  
      :param url:
          A unicode string of the URL to fetch the CRL from
  
      :param timeout:
          The number of seconds after which an HTTP request should timeout
  
      :return:
          An asn1crypto.crl.CertificateList object