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.
