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

@@ -1,86 +1,90 b''
 import sys
 import os
 from IPython.external.qt_for_kernel import QtCore, QtGui, enum_helper
 from IPython import get_ipython
 
 # If we create a QApplication, keep a reference to it so that it doesn't get
 # garbage collected.
 _appref = None
 _already_warned = False
 
 
 def _exec(obj):
     # exec on PyQt6, exec_ elsewhere.
     obj.exec() if hasattr(obj, "exec") else obj.exec_()
 
 
 def _reclaim_excepthook():
     shell = get_ipython()
     if shell is not None:
         sys.excepthook = shell.excepthook
 
 
 def inputhook(context):
     global _appref
     app = QtCore.QCoreApplication.instance()
     if not app:
         if sys.platform == 'linux':
             if not os.environ.get('DISPLAY') \
                     and not os.environ.get('WAYLAND_DISPLAY'):
                 import warnings
                 global _already_warned
                 if not _already_warned:
                     _already_warned = True
                     warnings.warn(
                         'The DISPLAY or WAYLAND_DISPLAY environment variable is '
                         'not set or empty and Qt5 requires this environment '
                         'variable. Deactivate Qt5 code.'
                     )
                 return
         try:
             QtCore.QApplication.setAttribute(QtCore.Qt.AA_EnableHighDpiScaling)
         except AttributeError:  # Only for Qt>=5.6, <6.
             pass
         try:
             QtCore.QApplication.setHighDpiScaleFactorRoundingPolicy(
                 QtCore.Qt.HighDpiScaleFactorRoundingPolicy.PassThrough
             )
         except AttributeError:  # Only for Qt>=5.14.
             pass
         _appref = app = QtGui.QApplication([" "])
 
         # "reclaim" IPython sys.excepthook after event loop starts
         # without this, it defaults back to BaseIPythonApplication.excepthook
         # and exceptions in the Qt event loop are rendered without traceback
         # formatting and look like "bug in IPython".
         QtCore.QTimer.singleShot(0, _reclaim_excepthook)
 
     event_loop = QtCore.QEventLoop(app)
 
     if sys.platform == 'win32':
         # The QSocketNotifier method doesn't appear to work on Windows.
         # Use polling instead.
         timer = QtCore.QTimer()
         timer.timeout.connect(event_loop.quit)
         while not context.input_is_ready():
             # NOTE: run the event loop, and after 50 ms, call `quit` to exit it.
             timer.start(50)  # 50 ms
             _exec(event_loop)
             timer.stop()
     else:
         # On POSIX platforms, we can use a file descriptor to quit the event
         # loop when there is input ready to read.
         notifier = QtCore.QSocketNotifier(
             context.fileno(), enum_helper("QtCore.QSocketNotifier.Type").Read
         )
         try:
             # connect the callback we care about before we turn it on
             # lambda is necessary as PyQT inspect the function signature to know
             # what arguments to pass to. See https://github.com/ipython/ipython/pull/12355
             notifier.activated.connect(lambda: event_loop.exit())
             notifier.setEnabled(True)
             # only start the event loop we are not already flipped
             if not context.input_is_ready():
                 _exec(event_loop)
         finally:
             notifier.setEnabled(False)
+
+    # This makes sure that the event loop is garbage collected.
+    # See issue 14240.
+    event_loop.setParent(None)