Add support for Django 6.0

[washeck]

Django 6.0 (currently in alpha, expected in December) changed the signature of Model._do_update method (returning_fields argument was added - see django/django@55a0073) causing django-concurrency to fail.

It can be reproduced by adding Django 6.0 to tox.ini

+    d60: django==6.0a1
+    d60: django-reversion==5.*

and running it with tox -e d60-sqlite

It would be nice to solve this issue before Django 6.0 is released.