Python / Django warn that your session is going to expire

You can extract the expiration date of the session from the template with:

{{ request.session.get_expiry_date }}

You only need a little bit of js to show the warning to the user when the date approaches.

But you can always use AJAX and create a view where you can check with js. In the view you would do something similar:

caducidad = request.session.get_expiry_date()