Django-iprestrict-redux

- Switched to Poetry (https://python-poetry.org/) + more cleanup
- Switched to pytest and pytest-django
- Drop support for pre-Django 1.10 style MIDDLEWARES
- Switch to old style url to path in urls.py
- Added Black and re-formatted and linted all the code
- Added isort and sorted all imports
- Bugfix: Add rule in admin was throwing an exception
- Handling "unknown" values in X-Forwarded-For header; introduces IPRESTRICT_USE_PROXY_IF_UNKNOWN header
- More cleanup

- General clean-up after forking the project from muccg/django-iprestrict
- Added support for Django 3+
- Dropped support for old releases of Django/Python