Upgrading from Qt5

[Robberduckzilla]

Re-opens PR-1425 from Benoit.

Summary of changes

With the Qt5 LTS support moved to commercial only, the only maintained version seems to be the KDE version, which is shipped by some distributions.

The first Q6 LTS release is also available, and there are now alternatives to PyQt5 / PyQt6: the official PySide2 / PySide6 bindings.

The 4 sets of bindings are mostly compatible.

By switching to QtPy, an abstraction layer for PyQt5/PyQt4/PySide2/PySide, and with a few compatibility fixes, we can target PyQt5, PySide2, PyQt6 and PySide6, making the transition to another set of bindings easier (and transparent for plugins).

Main changes:

• import from qtpy
• don't use QVariant when declaring signals (just use object).
• use scoped enums accesses: e.g. QDialogButtonBox.StandardButton.Ok instead of QDialogButtonBox.Ok. Necessary for Qt6 compatibility, increases our minimum supported version of PyQt5 to 5.11 (released in 2018).
• load resources directly from filesystem (no need for pyrcc5 anymore).
• use the QComboBox textActivated(QString) signal instead of activated(QString). Necessary for Qt6 compatibility,
• increases our minimum supported version of PyQt5 to 5.14 (released in 2019).
• a few other minor compatibility fixes for supporting PySide2 / PySide6 / PyQt6 / Qt6.

Known issues:

• the qt5-applications Windows' wheels are missing the rcc executable (reported Windows wheels are missing rcc.exe altendky/qt-applications#21).
• the official uic Python generator may generate invalid strings for some Unicode characters (like for this one in the plover_wpm_meter plugin).
• the dictionaries widget checkboxes don't work correctly with PyQt6 / PySide6

Pull Request Checklist

• Changes have tests
• News fragment added in news.d. See documentation for details

[Robberduckzilla]

Just tested this for a few hours and it definitely does not work as intended, needs more work before it's ready for PR.
I'll close this for now and raise another in future when it's ready.