Remove Django4.0 depreciation warning

[Camille-cmd]

Django 3.2 introduced an automatic AppConfig discovery. With automatic AppConfig discovery, default_app_config is no longer needed. As a consequence, it’s deprecated. This removes the default_app_config line in order to remove the warning.

All Submissions:

• [x] Are tests passing? (From the root-level of the repository please run pip install tox && tox)
• [ ] I have added or updated a test to cover the changes proposed in this Pull Request
• [ ] I have updated the documentation to cover the changes proposed in this Pull Request

[mbi]

Uhm, tests are failing on Django <= 3.1, if you'd like to pursue this PR, would mind taking a look if this is viable on all the supported Django versions?

[Camille-cmd]

Actually no and I apologize for not checking earlier! It is only valid from Django 3.2 and I might have mixed-up things as I though it was the case for django-rosetta, very sorry for that!