Template file incompatible with Django 3.0+

[cbahn]

I had trouble running this example on Django 3.1.7
It seems that the issue is with line 1 of booksite/templates/index.html
{% load staticfiles %} has been removed as of Django 3.0
Changing it to {% load static %} resolves the issue
If you would prefer to leave it as is, you may consider updating the readme to mention that the code is not compatible with Django 3.0+