builtins: improve builtins.function (#14094)

Remove two differences from types.FunctionType that should be
safe to clean up.

stdlib/builtins.pyi

@@ -1028,7 +1028,7 @@ class function:
     __annotations__: dict[str, AnnotationForm]
     if sys.version_info >= (3, 14):
         __annotate__: AnnotateFunc | None
-    __kwdefaults__: dict[str, Any]
+    __kwdefaults__: dict[str, Any] | None
     if sys.version_info >= (3, 10):
         @property
         def __builtins__(self) -> dict[str, Any]: ...
@@ -1036,6 +1036,26 @@ class function:
         __type_params__: tuple[TypeVar | ParamSpec | TypeVarTuple, ...]
 
     __module__: str
+    if sys.version_info >= (3, 13):
+        def __new__(
+            cls,
+            code: CodeType,
+            globals: dict[str, Any],
+            name: str | None = None,
+            argdefs: tuple[object, ...] | None = None,
+            closure: tuple[CellType, ...] | None = None,
+            kwdefaults: dict[str, object] | None = None,
+        ) -> Self: ...
+    else:
+        def __new__(
+            cls,
+            code: CodeType,
+            globals: dict[str, Any],
+            name: str | None = None,
+            argdefs: tuple[object, ...] | None = None,
+            closure: tuple[CellType, ...] | None = None,
+        ) -> Self: ...
+
     # mypy uses `builtins.function.__get__` to represent methods, properties, and getset_descriptors so we type the return as Any.
     def __get__(self, instance: object, owner: type | None = None, /) -> Any: ...