Update __new__ method to use Self type for improved type hinting (#918)

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: fleming79

traitlets/traitlets.py

@@ -1290,11 +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:
-        # Pass cls as args[0] to allow "cls" as keyword argument
-        cls = args[0]
-        args = args[1:]
-
+    def __new__(cls, /, *args: t.Any, **kwargs: t.Any) -> Self:
         # This is needed because object.__new__ only accepts
         # the cls argument.
         new_meth = super(HasDescriptors, cls).__new__
@@ -1305,13 +1301,10 @@ def __new__(*args: t.Any, **kwargs: t.Any) -> t.Any:
         inst.setup_instance(*args, **kwargs)
         return inst
 
-    def setup_instance(*args: t.Any, **kwargs: t.Any) -> None:
+    def setup_instance(self, /, *args: t.Any, **kwargs: t.Any) -> None:
         """
         This is called **before** self.__init__ is called.
         """
-        # Pass self as args[0] to allow "self" as keyword argument
-        self = args[0]
-        args = args[1:]
 
         self._cross_validation_lock = False
         cls = self.__class__
@@ -1333,11 +1326,7 @@ class HasTraits(HasDescriptors, metaclass=MetaHasTraits):
     _traits: dict[str, t.Any]
     _all_trait_default_generators: dict[str, t.Any]
 
-    def setup_instance(*args: t.Any, **kwargs: t.Any) -> None:
-        # Pass self as args[0] to allow "self" as keyword argument
-        self = args[0]
-        args = args[1:]
-
+    def setup_instance(self, /, *args: t.Any, **kwargs: t.Any) -> None:
         # although we'd prefer to set only the initial values not present
         # in kwargs, we will overwrite them in `__init__`, and simply making
         # a copy of a dict is faster than checking for each key.