Improve lib2to3 future feature annotation

Signed-off-by: Giridhar <80974392+giri256@users.noreply.github.com>

Author: giri256

stdlib/lib2to3/pytree.pyi

@@ -1,4 +1,4 @@
-from _typeshed import Incomplete, SupportsGetItem, SupportsLenAndGetItem, Unused
+from _typeshed import SupportsGetItem, SupportsLenAndGetItem, Unused
 from abc import abstractmethod
 from collections.abc import Iterable, Iterator, MutableSequence
 from typing import ClassVar, Final, TypeAlias
@@ -48,7 +48,7 @@ class Base:
 class Node(Base):
     fixers_applied: MutableSequence[BaseFix] | None
     # Is Unbound until set in refactor.RefactoringTool
-    future_features: frozenset[Incomplete]
+    future_features: frozenset[str]
     # Is Unbound until set in pgen2.parse.Parser.pop
     used_names: set[str]
     def __init__(