Refine return type for ManyToOneRel.get_accessor_name() (#2052)

Only for the special case of a symmetrical m2m relation on self,
`ForeignObjectRel.get_accessor_name()` will return `None`. However,
`ManyToOneRel` and its subclasses will never be symmetrical, so for
those classes `get_accessor_name()` does not return `None`.

django-stubs/db/models/fields/reverse_related.pyi

@@ -97,6 +97,7 @@ class ManyToOneRel(ForeignObjectRel):
         on_delete: Callable = ...,
     ) -> None: ...
     def get_related_field(self) -> Field: ...
+    def get_accessor_name(self, model: type[Model] | None = ...) -> str: ...
 
 class OneToOneRel(ManyToOneRel):
     field: OneToOneField