Update test app to use re_path (url deprecated on Django 4.0)

[sergioisidoro]

Tests were failing because Tox tests against main archive of Django, which is now on 4.x, and django.conf.urls.url() is removed. according to https://docs.djangoproject.com/en/4.0/releases/4.0/

[testenv:djangomain]
deps =
    https://github.com/django/django/archive/main.tar.gz#egg=django
    djangorestframework

Work towards #152

[dfunckt]

Interesting, I didn't realize django.conf.urls was deprecated since 2.0. Thanks for the PR!