Use Final for Constant Literals in the stdlib (#12332)

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

stdlib/multiprocessing/pool.pyi

@@ -1,7 +1,7 @@
 import sys
 from collections.abc import Callable, Iterable, Iterator, Mapping
 from types import TracebackType
-from typing import Any, Generic, Literal, TypeVar
+from typing import Any, Final, Generic, TypeVar
 from typing_extensions import Self
 
 if sys.version_info >= (3, 9):
@@ -97,7 +97,7 @@ class ThreadPool(Pool):
     ) -> None: ...
 
 # undocumented
-INIT: Literal["INIT"]
-RUN: Literal["RUN"]
-CLOSE: Literal["CLOSE"]
-TERMINATE: Literal["TERMINATE"]
+INIT: Final = "INIT"
+RUN: Final = "RUN"
+CLOSE: Final = "CLOSE"
+TERMINATE: Final = "TERMINATE"