Manager with generic QuerySet using TypeVar defaults

[UnknownPlatypus]

I have made things!

This is redo for #1270 using TypeVar defaults to implement the change in a non-breaking way.

This is not completely ready, I opening it to get some feedback and because there are still a few issues to address.

The PR currently passes tests but I've tried it on my work codebase and some related issues have resurfaced (not caused by this PR, but by the fact that a lot of places previously ignored are now checked)

Most notably:

1. ✅ .annotate() in custom queryset method looses type
   • How to correctly type manager functions that adds annotations? #2681
   • QuerySet annotations are lost when getting an instance in the queryset (bug with WithAnnotations) #1049
   • a bit related to Don't populate a type arg for non generic managers #2314
2. ✅ I had to do the same re-parametrization trick we do for Manager (cf Reparametrize managers without explicit type parameters #1169) which causes exactly the same kind of issue with overrides of models.Manager.get_queryset which are rather common because documented -> was done in Reparametrize implicit generic QuerySet subclasses #3217
3. ✅ Also Prefetch's to_attr raises "Model" has no attribute "prefetched_field" #795 was more present -- Fixed in Initial support for to_attr inference in Prefetch calls #2779)

I think this is step in the right direction but I'm a bit afraid it might cause some churn if we don't address the related issues first.

Todos

• Maybe fix Add test for missing typing import when used m2m to contrib auth #286 ?
• Maybe fixes Reverse dependency on django.contrib.auth.models.Permission breaks the from_queryset machinery #287 ?

Related issues

• Closes Making QuerySet classes part of the type of Manager classes #863 (tracking Issue)
• Closes WIP: Implement Managers with generic QuerySets #1270 (supersede prior pr addressing Making QuerySet classes part of the type of Manager classes #863)

Also fixes a bunch of related issues, I've added regression tests for them:

• Closes Custom queryset typing is lost #2602
• Closes Mypy can't get custom manager type #1067
• Closes Could not resolve manager type for "core.models.FooModel.objects" [django-manager-missing] #1023
• Closes select_related changes the manager class #728
• Closes Related Manager lose typing #1918

[UnknownPlatypus]

I needed to expose this symbol because I use it in the plugin.
If we don't want that, I can also add a type-ignore in the plugin code but I think it's fine to have it because it's a core component of a manager and very unlikely to disappear

[rtpg]

@UnknownPlatypus do you want eyes on this? I had issues with annotations that ended up with me writing this test, maybe this branch could end up resolving those too

[UnknownPlatypus]

@rtpg I plan on rebasing this pr latter this week now that most of the blocking issue mentioned in the description are resolved.

If you have some time, it would be great to resolve the annotations in custom queryset issue. Without that, the current state of this PR will cause a lot of issues for such cases that were previously ignored

I'll integrate the test you proposed in this MR, I think it will work