Add support for DjangoTask's delay_on_commit and apply_async_on_commit methods

[federicobond]

Celery 5.4 introduced some shortcuts for Django projects using a custom task class. This new DjangoTask base class adds the following methods:

• delay_on_commit
• apply_async_on_commit

https://docs.celeryq.dev/en/stable/django/first-steps-with-django.html#trigger-tasks-at-the-end-of-the-database-transaction

Not sure what's the best way to support app.task/shared_task returning instances of this base class but we can start by adding stubs for celery.contrib.django.task.DjangoTask (see API reference).

[federicobond]

Initial support for DjangoTask is in #173.

A small mypy plugin can fill in the gap by tweaking the return type of the shared_task and Celery.task decorators. @sbdchd, would you be interested in hosting such plugin in this repo if I submit a pull request?

Edit: this is a minimalistic version of the plugin.

[sbdchd]

I'm trying to keep these types free from mypy plugins so that pyright and similar can still function

how's the experience for non-mypy type checkers when the mypy plugin is setup?

[federicobond]

For non-mypy typecheckers, an explicit cast on the task would be needed to use the methods defined in DjangoTask. It does not affect the experience in any other way.