Support Django 3 and 4 and Python 3.8+

[n1ngu]

Any plans on officially supporting newer django and python versions?

I got object_tools working on Python 3.9 + Django 3.2 but the recent Django 4.0 finally broke my install due to

https://docs.djangoproject.com/en/4.0/releases/4.0/#features-removed-in-4-0

django.conf.urls.url() is removed.