Fix memory leak in Qt event loop integration (#14240) (#14251)

The QEventLoop object, `event_loop`, created in
`IPython/terminal/pt_intputhooks/qt.py` L58 is not deleted when exiting
the scope as passing `app` to the constructor parents the object to
`app`. This creates a memory leak as QEventLoop objects are being
accumulated.

The issue was originally reported by @pag who also suggested the fix.

`pytest IPython/terminal/tests/` runs through without errors. I have
tested the changes and see no more accumulation of `QEventLoop` objects.

This fixes  #14240

IPython/terminal/pt_inputhooks/qt.py

@@ -84,3 +84,7 b' def inputhook(context):'
                 _exec(event_loop)
         finally:
             notifier.setEnabled(False)
+
+    # This makes sure that the event loop is garbage collected.
+    # See issue 14240.
+    event_loop.setParent(None)