Add support for BaseManager.from_queryset() (#251)

* add support for BaseManager.from_queryset()

* cleanups

* lint fixes

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

@@ -0,0 +1,52 @@
+-   case: test_from_queryset_returns_intersection_of_manager_and_queryset
+    main: |
+        from myapp.models import MyModel, NewManager
+        reveal_type(NewManager())  # N: Revealed type is 'myapp.models.NewManager'
+        reveal_type(MyModel.objects)  # N: Revealed type is 'myapp.models.MyModel_NewManager[myapp.models.MyModel]'
+        reveal_type(MyModel.objects.get())  # N: Revealed type is 'Any'
+        reveal_type(MyModel.objects.manager_only_method())  # N: Revealed type is 'builtins.int'
+        reveal_type(MyModel.objects.manager_and_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
+                class ModelBaseManager(models.Manager):
+                    def manager_only_method(self) -> int:
+                        return 1
+                class ModelQuerySet(models.QuerySet):
+                    def manager_and_queryset_method(self) -> str:
+                        return 'hello'
+
+                NewManager = ModelBaseManager.from_queryset(ModelQuerySet)
+                class MyModel(models.Model):
+                    objects = NewManager()
+
+-   case: test_from_queryset_with_class_name_provided
+    main: |
+        from myapp.models import MyModel, NewManager
+        reveal_type(NewManager())  # N: Revealed type is 'myapp.models.NewManager'
+        reveal_type(MyModel.objects)  # N: Revealed type is 'myapp.models.MyModel_NewManager[myapp.models.MyModel]'
+        reveal_type(MyModel.objects.get())  # N: Revealed type is 'Any'
+        reveal_type(MyModel.objects.manager_only_method())  # N: Revealed type is 'builtins.int'
+        reveal_type(MyModel.objects.manager_and_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
+                class ModelBaseManager(models.Manager):
+                    def manager_only_method(self) -> int:
+                        return 1
+                class ModelQuerySet(models.QuerySet):
+                    def manager_and_queryset_method(self) -> str:
+                        return 'hello'
+
+                NewManager = ModelBaseManager.from_queryset(ModelQuerySet, class_name='NewManager')
+                class MyModel(models.Model):
+                    objects = NewManager()
+