Annotate sys.platform as LiteralString (#13378)

stdlib/sys/__init__.pyi

@@ -6,7 +6,7 @@ from collections.abc import AsyncGenerator, Callable, Sequence
 from io import TextIOWrapper
 from types import FrameType, ModuleType, TracebackType
 from typing import Any, Final, Literal, NoReturn, Protocol, TextIO, TypeVar, final, type_check_only
-from typing_extensions import TypeAlias
+from typing_extensions import LiteralString, TypeAlias
 
 _T = TypeVar("_T")
 
@@ -45,7 +45,7 @@ if sys.version_info >= (3, 10):
 path: list[str]
 path_hooks: list[Callable[[str], PathEntryFinderProtocol]]
 path_importer_cache: dict[str, PathEntryFinderProtocol | None]
-platform: str
+platform: LiteralString
 if sys.version_info >= (3, 9):
     platlibdir: str
 prefix: str