Intermittent end-of-process segfault on Linux + PySide 6

EDIT 2023-03-10: Upstream issue - https://bugreports.qt.io/browse/PYSIDE-2254

---

The script below _sometimes_ gives me an end-of-process segfault on Ubuntu 22.04 with PySide 6.3.2.

See the docstring at the top of the code for steps to reproduce.

Results of a typical failing run on my machine look like this:

```
(crasher) mdickinson@ubuntu-2204:~/Desktop$ python -m unittest crasher.py
/home/mdickinson/Desktop/crasher/lib/python3.10/site-packages/pyface/util/guisupport.py:155: DeprecationWarning: 'exec_' will be removed in the future. Use 'exec' instead.
app.exec_()
.
----------------------------------------------------------------------
Ran 1 test in 0.018s

OK
Segmentation fault
```

`faulthandler` gives no extra information, presumably because the segfault happens sufficiently late in the Python process teardown that faulthandler is no longer running.

```python
"""
The script below segfaults for me around 10% of the time with:

- Ubuntu 22.04 (running under VirtualBox on a macOS Ventura / Intel host)
- Python 3.10 (installed from official Ubuntu package)
- PySide 6.3.2 (installed via pip)

To reproduce:

- Save this script under the name 'crasher.py'
- Create and activate a Python 3.10 venv with e.g.

python -m venv --clear crasher
source crasher/bin/activate

- Install pyface and PySide6 < 6.4 from PyPI:

python -m pip install pyface "PySide6<6.4"

- Run this script under unittest:

python -m unittest crasher.py

"""

import unittest

from pyface.gui import GUI
from pyface.tasks.api import TaskWindow
from traits.api import HasTraits, Instance

class MyTasksApplication(HasTraits):
window = Instance(TaskWindow)

def run(self):
gui = GUI()
window = TaskWindow()
window.open()
self.window = window
gui.invoke_later(self.exit)
gui.start_event_loop()

def exit(self):
window = self.window
self.window = None
window.destroy()
window.closed = True

class TestTasksApplication(unittest.TestCase):
def test_lifecycle(self):
app = MyTasksApplication()
app.run()
```

Start with the reproduction steps and crasher.py in the issue, then inspect the traceback reference to pyface/util/guisupport.py and compare the behavior with upstream PYSIDE-2254. Done would require a clear diagnosis and an actionable resolution, but the report indicates that the underlying work has moved to the upstream issue.

Written by the indexing model from the issue text.