Update the output type for the __new__ function in Model (bokeh#13564)

* Update the output type for the __new__ function in Model

This fixes an issue with typecheckers for detecting the return value of creating a Model

* Update model.py to not use any return type for __new__

* Update model.py to use self again

* Update for isort issues

src/bokeh/model/model.py

@@ -39,6 +39,8 @@
 from .util import HasDocumentRef, collect_models, visit_value_and_its_immediate_references
 
 if TYPE_CHECKING:
+    from typing_extensions import Self
+
     from ..core.has_props import Setter
     from ..core.query import SelectorType
     from ..document import Document
@@ -87,7 +89,7 @@ def __init_subclass__(cls):
 
     _id: ID
 
-    def __new__(cls, *args: Any, id: ID | None = None, **kwargs: Any) -> Model:
+    def __new__(cls, *args: Any, id: ID | None = None, **kwargs: Any) -> Self:
         obj = super().__new__(cls)
 
         # Setting 'id' implies deferred initialization, which means properties