Tests pass for PyQt5 but not PySide2 or PySide6

[hugovk]

What did you do?

python3 -m pip uninstall pyqt5 pyside2 -y
python3 -m pip install pyqt5  # currently 5.15.2
pytest Tests/test_*qt*
# Tests pass 👍

python3 -m pip uninstall pyqt5 pyside2 -y
python3 -m pip install pyside2  # currently 5.15.2
pytest Tests/test_*qt*
# Tests fail 👎 

What did you expect to happen?

Tests pass.

What actually happened?

...
Tests/test_imageqt.py ..                                                                                                                 [ 33%]
Tests/test_qt_image_fromqpixmap.py .                                                                                                     [ 50%]
Tests/test_qt_image_toqimage.py .F                                                                                                       [ 83%]
Tests/test_qt_image_toqpixmap.py E                                                                                                       [100%]

==================================================================== ERRORS ====================================================================
_________________________________________________ ERROR at setup of TestToQPixmap.test_sanity __________________________________________________

self = <class 'Tests.test_qt_image_toqpixmap.TestToQPixmap'>

    @classmethod
    def setup_class(self):
        try:
            if ImageQt.qt_version == "5":
                from PyQt5.QtGui import QGuiApplication
            elif ImageQt.qt_version == "side2":
                from PySide2.QtGui import QGuiApplication
        except ImportError:
            pytest.skip("QGuiApplication not installed")
            return

>       self.app = QGuiApplication([])
E       RuntimeError: Please destroy the QGuiApplication singleton before creating a new QGuiApplication instance.

Tests/test_imageqt.py:24: RuntimeError
=================================================================== FAILURES ===================================================================
________________________________________________________________ test_segfault _________________________________________________________________

    def test_segfault():
>       app = QApplication([])
E       RuntimeError: Please destroy the QGuiApplication singleton before creating a new QApplication instance.

Tests/test_qt_image_toqimage.py:55: RuntimeError
=========================================================== short test summary info ============================================================
ERROR Tests/test_qt_image_toqpixmap.py::TestToQPixmap::test_sanity - RuntimeError: Please destroy the QGuiApplication singleton before creati...
FAILED Tests/test_qt_image_toqimage.py::test_segfault - RuntimeError: Please destroy the QGuiApplication singleton before creating a new QApp...
===================================================== 1 failed, 4 passed, 1 error in 0.82s =====================================================

What are your OS, Python and Pillow versions?

• OS: macOS Mojave
• Python: 3.9.1
• Pillow: master

A similar thing happens with PySide6 (6.0.0, once support added to Pillow).

We're only testing PyQt5 on the CI.

[radarhere]

I've created PR #5167 as a possible solution.