Django-rest-knox

- compatibility with Python up to 3.10 and Django up to 4.0
- integration with github CI instead of travis
- Migration: "salt" field of model "AuthToken" is removed

- Expiry format now defaults to whatever is used Django REST framework
- The behavior can be overridden via EXPIRY_DATETIME_FORMAT setting
- Fully customizable expiry format via format_expiry_datetime
- Fully customizable response payload via get_post_response_data

- Fix for tox config to build Django 2.2 on python 3.6

**BREAKING** This is a major release version because it
breaks the existing API.
Changes have been made to the `create()` method on the `AuthToken` model.
It now returns the model instance and the raw `token` instead
of just the `token` to allow the `expiry` field to be included in the
success response.

Model field of `AuthToken` has been renamed from `expires` to `expiry`
to remain consistent across the code base. This patch requires you
to run a migration.

Depending on your usage you might have to adjust your code
to fit these new changes.

- `AuthToken` model field has been changed from `expires` to `expiry`
- Successful login now always returns a `expiry` field for when the token expires

- The user serializer for each `LoginView`is now dynamic

- The context, token TTL and tokens per user settings in `LoginView` are now dynamic