restore check for readline

fix regression in debugger exit for non-readline contexts

IPython/core/debugger.py

@@ -288,6 +288,7 b' class Pdb(OldPdb):'
                 break
             finally:
                 # Pdb sets readline delimiters, so set them back to our own
+                if self.shell.readline is not None:
                 self.shell.readline.set_completer_delims(self.shell.readline_delims)
 
     def new_do_up(self, arg):