Add inheritance QuerySet support for from_queryset (#275)

* Add testcase for queryset inheritance

* Add PoC

* Add condition for stop to looping over mro

* Change harcoded queryset class name to constant from fullnames

test-data/typecheck/managers/querysets/test_from_queryset.yml

@@ -94,3 +94,26 @@
                 class MyModel(models.Model):
                     objects = NewManager()
 
+-   case: from_queryset_with_class_inheritance
+    main: |
+        from myapp.models import MyModel
+        reveal_type(MyModel().objects)  # N: Revealed type is 'myapp.models.MyModel_NewManager[myapp.models.MyModel]'
+        reveal_type(MyModel().objects.get())  # N: Revealed type is 'myapp.models.MyModel*'
+        reveal_type(MyModel().objects.queryset_method())  # N: Revealed type is 'builtins.str'
+    installed_apps:
+        - myapp
+    files:
+        - path: myapp/__init__.py
+        - path: myapp/models.py
+          content: |
+              from django.db import models
+              from django.db.models.manager import BaseManager
+              class BaseQuerySet(models.QuerySet):
+                  def queryset_method(self) -> str:
+                      return 'hello'
+              class ModelQuerySet(BaseQuerySet):
+                  pass
+
+              NewManager = BaseManager.from_queryset(ModelQuerySet)
+              class MyModel(models.Model):
+                  objects = NewManager()