Fix quitting debugger when readline not in use

Closes gh-8465

IPython/core/debugger.py

@@ -303,7 +303,8 b' class Pdb(OldPdb):'
             self.shell.Completer.all_completions=self.old_all_completions
 
         # Pdb sets readline delimiters, so set them back to our own
-        self.shell.readline.set_completer_delims(self.shell.readline_delims)
+        if self.shell.readline is not None:
+            self.shell.readline.set_completer_delims(self.shell.readline_delims)
 
         return OldPdb.do_quit(self, arg)