    def init_server_extension_config(self):
        """
        Overloads a method in classic notebook server's NotebookApp class
        (notebook < 7) to conditionally enable a jupyterhub test extension.

        ref: https://github.com/jupyter/notebook/blob/v6.5.2/notebook/notebookapp.py#L1982
        """
        super().init_server_extension_config()
        if os.getenv("JUPYTERHUB_SINGLEUSER_TEST_EXTENSION") == "1":
            self.log.warning("Enabling jupyterhub test extension, classic edition")
            self.nbserver_extensions["jupyterhub.tests.extension"] = True
Overloads a method in classic notebook server's NotebookApp class
(notebook < 7) to conditionally enable a jupyterhub test extension.

ref: https://github.com/jupyter/notebook/blob/v6.5.2/notebook/notebookapp.py#L1982
jupyterhub/singleuser/mixins.py