Fix for PyQt6

[hugovk]

Fixes #5774.

There are two problems in #5774:

• First (Qt test failures #5774 (comment)) is a possibly intermittent failure on the CI on release day seems to have resolved itself and still sometimes happening. It's possibly an issue with the MSYS2/MinGW build of Qt.

• Second (Qt test failures #5774 (comment)) is a failure with PyQt 6.2.0 on Windows

This PR fixes the second,

We've only been testing PyQt5 on the CI. I can also reproduce this locally using PyQt6 on macOS, and on CI with MinGW (https://github.com/hugovk/Pillow/runs/3915553306?check_suite_focus=true), so platform is irrelevant.

We added PyQt6 support in #5258, but as far as I can tell that should never have worked:

I compared the sdist of PyQt 6.0.0 with 6.0.3 (latest 6.0.x), and 6.1.1 (latest 6.1.x) and 6.2.0 (latest 6.2.x) and didn't see anything much changing in qiodevice* files.

So I don't know if we really need QIODevice.OpenMode from #5258. If no-one else is sure, guess it's okay to keep it in the try/except.

This also tests PyQt6 on the CI, on MinGW, to hopefully fix the first part, and means we're now testing both PyQt5 and 6 on CI.

[radarhere]

It's not clear why I chose OpenMode in #5258. Perhaps I just misunderstood https://doc.qt.io/qt-5/qiodevice.html#OpenModeFlag-enum ?

Happy for it to be left in or removed.

[hugovk]

Something has actually changed since PyQt6 6.0.0 and 6.2.0, and the old code is still needed.

On Mac, this fails on main and passes on this PR:

pip uninstall -y pyqt6; pip install pyqt6==6.2.0; pytest Tests/test_image_fromqimage.py

And this passes for both main and this PR:

pip uninstall -y pyqt6; pip install pyqt6==6.0.0; pytest Tests/test_image_fromqimage.py