Admin site doesn't show left panel

Admin site for categories doesn't show left panel introduced in Django 3.0:
![Snímek obrazovky_2021-12-17_09-08-48](https://user-images.githubusercontent.com/156755/146511781-12df2f40-cee1-4dbd-a8ee-2441b768ec20.png)
vs
![Snímek obrazovky_2021-12-17_09-09-33](https://user-images.githubusercontent.com/156755/146511784-bcd68d23-d31d-4a81-93b9-fb3f891966db.png)
