Always use test settings in pytest

You can alternatively use DJANGO_SETTINGS_MODULE, but this is overridden
by the environment variable of the same name. In order to always run it
with test settings, regardless of the environment variable use --ds.

https://pytest-django.readthedocs.io/en/latest/configuring_django.html

{{cookiecutter.project_slug}}/pytest.ini

@@ -1,5 +1,5 @@
 [pytest]
-DJANGO_SETTINGS_MODULE=config.settings.test
+addopts = --ds=config.settings.test
 {%- if cookiecutter.js_task_runner != 'None' %}
 norecursedirs = node_modules
 {%- endif %}