Helper method to return the Session expiration time.

        If the session is not 'permanent' it will expire as and when
        the browser stops accessing the app.

def get_expiration_time(self, app: 'Quart', session: SessionMixin) -> Optional[datetime]:
        """Helper method to return the Session expiration time.

        If the session is not 'permanent' it will expire as and when
        the browser stops accessing the app.
        """
        if session.permanent:
            return datetime.utcnow() + app.permanent_session_lifetime
        else:
            return None