refactor: specify that clear_all is an abstract method

Co-authored-by: Donghee Na <donghee.na92@gmail.com>

Author: sharmaigne

Lib/_pyrepl/console.py

@@ -114,6 +114,7 @@ def clear(self) -> None:
         """Wipe the screen"""
         ...
 
+    @abstractmethod
     def clear_all(self) -> None:
         """Clear screen and scrollback buffer."""
         ...