* 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.
