Django 6.0: Add Django Tasks framework

[federicobond]

I wrote stubs for the Django Task framework introduced in Django 6.0. They still need some polish but feedback is very welcome.

If you are reviewing this, pay special attention to the Task class generic parameters which have an influence in how many other methods are typed.

Update: this now passes all tests, I had to put some items in the allowlist and conditionally monkeypatch the tasks classes based on the Django version. There are some rough edges there, but things are looking good.

[sobolevn]

Question: why are these items in todo.txt?

• If these are expected errors, move them to regular allowlist.txt (without todo) and comment why they are here
• If they are just not fixed yet, what is blocking us from fixing them?

[federicobond]

Honestly because I wasn't aware of how to use each list, so I just threw those items back to where they came from.

• django.tasks.default_task_backend could be moved to allowlist_todo.txt with the other ConnectionProxy instances, like django.core.cache.cache and django.db.connection.
• django.tasks.task decorator emits an error because one of the overloads expects a non-None function value when the decorator accepts None at runtime (for the other overload). Could move it to allowlist.txt
• django.tasks.backends.base.BaseTaskBackend.enqueue should be typed as an abstract method but not sure if that's going to be problematic since default_task_backend is typed as BaseTaskBackend. Can move it to allowlist.txt but not sure.