Fix deprecation warnings

[knokko]

This pull requests fixes all linting issues and Django 3 deprecation warnings. A branch of SMS is already using this and it looks like nothing broke.

[knokko]

The following deprecation warnings are fixed:

/usr/local/lib/python3.7/dist-packages/binder/router.py:174: RemovedInDjango40Warning: django.conf.urls.url() is deprecated in favor of django.urls.re_path().

/usr/local/lib/python3.7/dist-packages/binder/history.py:14: 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.