code: def _generate_appsecret_proof(self):
          
          if six.PY2:
              key = self.appsecret
              message = self.oauth_token
          else:
              key = bytes(self.appsecret, 'utf-8')
              message = bytes(self.oauth_token, 'utf-8')
  
          return hmac.new(key, message, hashlib.sha256).hexdigest()
docs: Returns a SHA256 of the oauth_token signed by appsecret.
          https://developers.facebook.com/docs/graph-api/securing-requests/