text: Title: Django40 deprecation warnings
  Body: **Describe the bug**
  When upgrading to ue Django 3.1.2, the following warnings are emitted:
  ```
    /usr/local/lib/python3.8/dist-packages/import_export/admin.py:368: RemovedInDjango40Warning: django.conf.urls.url() is deprecated in favor of django.urls.re_path().
      url(r'^export/$',
    /usr/local/lib/python3.8/dist-packages/import_export/signals.py:3: RemovedInDjango40Warning: The providing_args argument is deprecated. As it is purely documentational, it has no replacement. If you rely on this argument as documentation, you can move the text to a code comment or docstring.
      post_export = Signal(providing_args=["model"])
  ```
  
  **To Reproduce**
  Steps to reproduce the behavior:
  1. Upgrade Django to 3.1.2.
  2. Run some tests which invoke Django views:
      ```
      py.test -slv test_with_django_view.py
      ```
  3. Scroll down to 'warnings summary'
  4. See messages
  
  **Versions**
   - Django Import Export: 2.3.0
   - Python 3.8
   - Django 3.1.2
  
  **Expected behavior**
  No deprecation warning should be emitted.
  