updating Qt libraries

[jjnurminen]

PyQt5 end of life is coming in mid-2023. At some point, it's time to move to PyQt6. (It's almost 2 years since it was released, but still no conda packages?) Needs updates to requirements, import statements and possibly minor code changes too.

Alternatively, investigate if it's feasible to move to PySide2, which is the official Qt for Python binding. Last time I checked I didn't want to do it, since Qt Group was being tricky with licensing or something.

(It's also possible to support both PySide2 and PyQt by conditional import statements and some other minor trickery, but this is probably not necessary)