This allows CrowdAnki on Anki 23.10rc1 to run without
ENABLE_QT5_COMPAT=1.

I believe that compatibility with Qt5 and Anki 2.1.50->2.1.66 is
preserved.

The Qt6 version of the code is in the try clause, while the Qt5 one is
in the except.  Hopefully, this will make clean up (once Qt5 is
dropped by Anki) simple.

There are still some Qt-unrelated deprecation warning with 23.10rc1,
but it's probably cleanest to deal with them separately, since I'm not
sure how easy it will be to make the relevant changes
backwards-compatible.

After explicitly testing, it seems that the "Qt6" versions also seem
to just work for Qt5 (at least going back down to 2.1.50), so there's
no need for the two options.

It seems that exec_ was needed for python2 compat:

https://stackoverflow.com/questions/22610720/should-i-use-app-exec-or-app-exec-in-my-pyqt-application

(PyQt6 only supports exec; PyQt5 supports both exec and exec_.)

I'm keeping the previous commit unsquashed (for now?), for reference,
just in case something is broken here.

This allows changing note type and opening the config window to work.

I think that this is now all the breakage, for now.

Compat with 2.1.50+ is AFAICT preserved.