Refuse to install event loop hooks when not using `prompt_toolkit`

Without this, `%gui` is effectively a no-op but the user thinks it works.

IPython/terminal/interactiveshell.py

@@ -914,6 +914,11 b' class TerminalInteractiveShell(InteractiveShell):'
 
     active_eventloop = None
     def enable_gui(self, gui=None):
+        if self.simple_prompt is True and gui is not None:
+            print(f"Cannot install event loop hook for \"{gui}\" when running with `--simple-prompt`.")
+            print("NOTE: Tk is supported natively; use Tk apps and Tk backends with `--simple-prompt`.")
+            return
+        
         if self._inputhook is None and gui is None:
             print("No event loop hook running.")
             return