give PTCompleter InteractiveShell, not Completer

it is possible for the completer to get reloaded/replaced,
at which point the prompt-toolkit completions will not be those of ip.Completer,
but whatever ip.Completer was originally.

IPython/terminal/interactiveshell.py

@@ -238,7 +238,7 b' class TerminalInteractiveShell(InteractiveShell):'
                             editing_mode=editing_mode,
                             key_bindings_registry=kbmanager.registry,
                             history=history,
-                            completer=IPythonPTCompleter(self.Completer),
+                            completer=IPythonPTCompleter(self),
                             enable_history_search=True,
                             style=style,
                             mouse_support=self.mouse_support,

IPython/terminal/ptutils.py

@@ -12,8 +12,12 b' import pygments.lexers as pygments_lexers'
 
 class IPythonPTCompleter(Completer):
     """Adaptor to provide IPython completions to prompt_toolkit"""
-    def __init__(self, ipy_completer):
-        self.ipy_completer = ipy_completer
+    def __init__(self, shell):
+        self.shell = shell
+    
+    @property
+    def ipy_completer(self):
+        return self.shell.Completer
 
     def get_completions(self, document, complete_event):
         if not document.current_line.strip():