py-django-registration: updated to 2.3

In django-registration 2.3, the new validators :func:`~registration.validators.validate_confusables` and :func:`~registration.validators.validate_confusables_email` were added, and are applied by default to the username field and email field, respectively, of registration forms. This may cause some usernames which previously were accepted to no longer be accepted, but like the reserved-name validator this change was made because its security benefits significantly outweigh the edge cases in which it might disallow an otherwise-acceptable username or email address. If for some reason you need to allow registration with usernames or email addresses containing potentially dangerous use of Unicode, you can subclass the registration form and remove these validators, though doing so is not recommended.

(adam)