stub fix

Author: DavidCEllis

src/ducktools/classbuilder/__init__.pyi

@@ -47,7 +47,7 @@ def build_completed(cls: type) -> bool: ...
 def _get_inst_fields(inst: typing.Any) -> dict[str, typing.Any]: ...
 
 class _NothingType:
-    def __init__(self, custom: str | None = ...) -> None: ...
+    def __new__(cls, custom: str | None = ...) -> typing.Self: ...
     def __repr__(self) -> str: ...
 NOTHING: _NothingType
 FIELD_NOTHING: _NothingType