[stdlib][sys] Change return types to use Literal/LiteralString (#15032)

Author: guoci

stdlib/sys/__init__.pyi

@@ -378,13 +378,13 @@ if sys.platform == "android":  # noqa: Y008
     def getandroidapilevel() -> int: ...
 
 def getallocatedblocks() -> int: ...
-def getdefaultencoding() -> str: ...
+def getdefaultencoding() -> Literal["utf-8"]: ...
 
 if sys.platform != "win32":
     def getdlopenflags() -> int: ...
 
-def getfilesystemencoding() -> str: ...
-def getfilesystemencodeerrors() -> str: ...
+def getfilesystemencoding() -> LiteralString: ...
+def getfilesystemencodeerrors() -> LiteralString: ...
 def getrefcount(object: Any, /) -> int: ...
 def getrecursionlimit() -> int: ...
 def getsizeof(obj: object, default: int = ...) -> int: ...