Move to newer urls function to avoid Django 4.0 deprecation warning

When running the test suite there are a number of warnings of the form
```
demo/tests/test_dpd_demo.py::test_template_tag_use
  /home/mark/local/django-plotly-dash/demo/demo/urls.py:38: RemovedInDjango40Warning:
```

These can be removed by modifying the code to use the newer functions.
