Draft: Django 3 Upgrade

Ishaan Arora requested to merge pulsar17/inkscape-web:django-3-upgrade into master

This MR has the patches to make inkscape-web to be able to be upgraded to Django 3.x.

Not many things have been changed, mostly deprecations and some removals as per the release notes.

However, I'll also include some patches that will make eventual transition to Django 4.x easy because why not :) (mostly things that were introduced in 2.x/3.x and are removed in 4.x)

Merge request reports