* 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.
This addresses an obscure crash we're getting when defining a GenericForeignKey
subclass on a model.
Not sure how this slipped through type checking since
`helpers.lookup_class_typeinfo -> Optional[TypeInfo]` while
`.get_private_descriptor_type(type_info: TypeInfo)` so this should be a clear
type violation.
Prior to this change, ManyToManyField was declared as being generic in
_ST and _GT, but also used the _T Typevar in its __init__ signature.
This caused mypy to add _T to the variables it was generic in when used
as an alias.
The symptom of this problem was that mypy would show an error with the
message "Type application has too few types (3 expected)" where a
ManyToManyField alias was declared, but adding an extra argument would
fail because the type only takes two arguments.
This change brings the signature of ManyToManyField in line with
ForeignKey and OneToOneField.
