Work on the qt6 only world is on hold

[Edward K. Ream]

I have closed PR #2883 for now. Its milestone is "Later."

As shown on Leo's branches page, the ekr-qt6-only branch is protected but closed.  I'll reopen this branch when Leo requires Qt6, which might not happen for years.

It was straightforward to require Qt6. One simply looks for the isQt5 and isQt6 constants.

Requiring Qt6 helped pylint a bit. Alas, focusing on Qt6 did not help mypy much. I abandoned the PR (for now) when I realized it would be better to suppress mypy complaints only when Leo moves to the Qt6-only code base. That way I can deal with potential bugs more fully.

Summary

Yesterday's work will eventually bear fruit, but the results were disappointing.

I see no need to require Qt6 this year or next.

Edward