Adds Literal and __all__ to cmd.pyi (#6847)

stdlib/cmd.pyi

@@ -1,4 +1,10 @@
 from typing import IO, Any, Callable
+from typing_extensions import Literal
+
+__all__ = ["Cmd"]
+
+PROMPT: Literal["(Cmd) "]
+IDENTCHARS: str  # Too big to be `Literal`
 
 class Cmd:
     prompt: str