Update __new__ method to use Self type for improved type hinting

Author: fleming79

traitlets/traitlets.py

@@ -1290,7 +1290,7 @@ def class_init(self, cls: type[HasTraits], name: str | None) -> None:
 class HasDescriptors(metaclass=MetaHasDescriptors):
     """The base class for all classes that have descriptors."""
 
-    def __new__(*args: t.Any, **kwargs: t.Any) -> t.Any:
+    def __new__(*args: t.Any, **kwargs: t.Any) -> Self: # type:ignore[misc, type-var]
         # Pass cls as args[0] to allow "cls" as keyword argument
         cls = args[0]
         args = args[1:]
@@ -1303,7 +1303,7 @@ def __new__(*args: t.Any, **kwargs: t.Any) -> t.Any:
         else:
             inst = new_meth(cls, *args, **kwargs)
         inst.setup_instance(*args, **kwargs)
-        return inst
+        return inst # type:ignore[no-any-return]
 
     def setup_instance(*args: t.Any, **kwargs: t.Any) -> None:
         """