Rewrite to incorporate __class_getitem__ present and other stuff

[mkurnikov]

No description provided.

[mkurnikov]

Ok, this is what I imagined refactoring would be like.

Those are some descriptions of the PR:

1. Drop support for the Manager and QuerySet without annotations. So, for your

class MyModel(models.Model):
    mymanager = MyManager()

you'll have to explicitly change the manager's instantiation to the MyManager[MyModel], otherwise typechecker will give an error.

Django 3.1 will have native support for that via django/django@578c03b
For Django 2.2 and 3.0 user would need to use this helper https://github.com/typeddjango/django-stubs/pull/366/files#diff-cd31ac9ed2da033b1967049827559013R1

from mypy_django_plugin.lib import make_classes_generic
make_classes_generic(MyManager)

class MyModel(models.Model):
    mymanager = MyManager['MyModel']()

2. For the reason above, testing for both 2.2 and 3.0 test suites will be dropped. Stubs compatibility should be tested with the actual Django code, using
   https://github.com/ambv/retype
   https://github.com/google/pytype/tree/master/pytype/tools/merge_pyi
   or custom stubs merger based on https://libcst.readthedocs.io/en/latest/codemods.html#libcst.codemod.visitors.ApplyTypeAnnotationsVisitor
   (that's for @kszmigiel, I won't have the bandwidth)

3. This PR drops support for the Manager.from_queryset() and Queryset.as_manager(). Not because it's not implementable, just I don't have enough bandwidth to finish it. I imagine @kszmigiel would implement those on top of this PR.

4. API for plugin callbacks will be class-based, see https://github.com/typeddjango/django-stubs/pull/366/files#diff-a27b71ebfd9f3481ef4c189ac74fe6d7R9 for an example.

Before I move all the remaining code to the new API, I'd like to have others confirm/reject this proposal.

[sobolevn]

You have my approval 👍

Thanks a lot for working on this!
Amazing!

[sobolevn]

@kszmigiel you can take a look and get familiar with the code when you have the time!

[sobolevn]

@mkurnikov can you please rebase this branch on current master?