Django: Use path funcion instead of url funcion

[r-lelis]

The django.conf.urls.url function has been deprecated since version 3.1 and wil be removed on version 4. I replaced it with the easier to read path function.

Edit: I also added the support for Django 4.0 on tox and on Travis. I think this PR is important to make sure that people can update to later versions of Django without break anything.

[r-lelis]

I'm having some problems triggering Travis's tests @toastdriven @seocam

[galileoguzman]

This is a really good thing to have for those whom use Django 4. Is there any I can do?