[horizon] Support for Django 3.x

Hi,

It's been a long time that Django 3.x is around, and therefore, it'd be nice if Horizon had some kind of support for it. Indeed, sooner or later, Django 3.x will be uploaded to Debian unstable. That day, I hope Horizon doesn't just break.

So, what's the status? Is anyone is at least working on it?

Cheers,

Thomas Goirand (zigo)