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

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

stdlib/cmd.pyi

@@ -1,9 +1,9 @@
 from collections.abc import Callable
-from typing import IO, Any, Literal
+from typing import IO, Any, Final
 
 __all__ = ["Cmd"]
 
-PROMPT: Literal["(Cmd) "]
+PROMPT: Final = "(Cmd) "
 IDENTCHARS: str  # Too big to be `Literal`
 
 class Cmd: