ekr-qt-2 branch merged into devel

[Edward K. Ream]

The new code is a milestone in Leo's codebase:

- Leo now supports pyQt6.

- leoQt.py simplifies access to Qt constants, hiding details of the differences between pyQt5 and pyQt6.

- Leo no longer supports pyQt4.

I have been using the new code without problems for several days. All tests pass, including pylint and mypy.

Two issues remain for Leo 6.4b1:

- #2005 has already been fixed, but I'm going to "generalize" it so that leoQt6.py at least attempts to import all the modules that leoQt5.py imports.

- #2024 may be more problematic. In any case, this issue will not block Leo 6.4b1.

Summary

Please test devel and report any problems immediately.

I'll wait at least three more days until releasing Leo 6.4b1.

Edward