docs: def http_signature(message, key_id, signature):
      """Return a tuple (message signature, HTTP header message signature)."""
      template = ('Signature keyId="%(keyId)s",algorithm="hmac-sha256",'
                  'headers="%(headers)s",signature="%(signature)s"')
      headers = ['(request-target)', 'host', 'accept', 'date']
      return template % {
          'keyId': key_id,
          'signature': signature,
          'headers': ' '.join(headers),
      }