Use Final for unittest constants (#12447)

stdlib/unittest/result.pyi

@@ -2,14 +2,14 @@ import sys
 import unittest.case
 from _typeshed import OptExcInfo
 from collections.abc import Callable
-from typing import Any, TextIO, TypeVar
+from typing import Any, Final, TextIO, TypeVar
 from typing_extensions import TypeAlias
 
 _F = TypeVar("_F", bound=Callable[..., Any])
 _DurationsType: TypeAlias = list[tuple[str, float]]
 
-STDOUT_LINE: str
-STDERR_LINE: str
+STDOUT_LINE: Final[str]
+STDERR_LINE: Final[str]
 
 # undocumented
 def failfast(method: _F) -> _F: ...