Make AddRelatedManagers look for "objects" on parent model (#730)

* Add failing test for relation to model inheriting `objects`

Fails with:
```
pytest_mypy_plugins.utils.TypecheckAssertionError: Invalid output:
Expected:
  main:2: note: Revealed type is "myapp.models.MyUser*"
  main:3: note: Revealed type is "myapp.models.MyUser*"
  <45 (diff)
  <45 (diff)
Actual:
  main:2: note: Revealed type is "myapp.models.MyUser*"
  main:3: note: Revealed type is "myapp.models.MyUser*"
  main:6: error: "MyUser" has no attribute "book_set" (diff)
  main:6: note: Revealed type is "Any"          (diff)
  main:7: error: "MyUser" has no attribute "article_set" (diff)
  main:7: note: Revealed type is "Any"          (diff)
```

* Make AddRelatedManagers look for "objects" on parent model

Previously, AddRelatedManagers would fail if a related model had inherited
its `objects` field from a parent class. This would result in missing
relation attributes. This is fixed by using `get()` instead of `names`;
the former searches the MRO for the symbol, whereas the latter only looks
for symbols declared directly on the class.

tests/typecheck/fields/test_related.yml

@@ -601,6 +601,37 @@
                     pass
 
 
+-   case: test_foreign_key_from_superclass_inherits_correctly_when_also_inheriting_manager
+    main: |
+        from myapp.models import MyUser, Book, Article, LibraryEntity
+        reveal_type(Book().registered_by_user)  # N: Revealed type is "myapp.models.MyUser*"
+        reveal_type(Article().registered_by_user)  # N: Revealed type is "myapp.models.MyUser*"
+
+        user = MyUser()
+        reveal_type(user.book_set) # N: Revealed type is "django.db.models.manager.RelatedManager[myapp.models.Book]"
+        reveal_type(user.article_set) # N: Revealed type is "django.db.models.manager.RelatedManager[myapp.models.Article]"
+    installed_apps:
+        - myapp
+    files:
+        -   path: myapp/__init__.py
+        -   path: myapp/models.py
+            content: |
+              from django.db import models
+              class MyUser(models.Model):
+                  pass
+              class LibraryEntityQuerySet(models.QuerySet):
+                  pass
+              class LibraryEntity(models.Model):
+                  class Meta:
+                      abstract = True
+                  objects = models.Manager.from_queryset(LibraryEntityQuerySet)()
+                  registered_by_user = models.ForeignKey(MyUser, on_delete=models.CASCADE)
+              class Book(LibraryEntity):
+                  pass
+              class Article(LibraryEntity):
+                  pass
+
+
 -   case: foreign_key_relationship_for_models_with_custom_manager
     main: |
         from myapp.models import Transaction