Upgrade QT library to pyside6

[cmutel]

Feature request

Allows for more modern Python versions and could make AB pip-installable

[mrvisscher]

Relevant issues:

• qt6-webengine conda-forge/qt-webengine-feedstock#42
• How to get QtWebEngineWidgets on PySide6? conda-forge/pyside2-feedstock#167

[jsvgoncalves]

#11

[cmutel]

@jsvgoncalves Some context here: @mrvisscher already looked into this, and while everything in your PR would work for a pip-installable package, it would break in the other direction, as there is no Anaconda build of QT web engine for pyside6 available yet. There is a WIP PR on this but it seems to be going slowly.

We definitely don't want to have two forks of the AB, one only installable via pip and the other only installable via conda, so for now I think we have to either support the conda-forge maintainers or just be patient.