Add toolkit support for PySide6

[mdickinson]

We'll eventually want to support Qt 6 and PySide 6. PySide6 and shiboken6 are already released to PyPI.

Adding basic toolkit support for PySide6 is straightforward: it simply involves adding the "pyside6" option for QT_API, and making it behave in the same way as "pyside2". From experimentation, this is sufficient to get basic applications working under PySide6, but we'll need to be prepared for minor compatibility differences.

[rahulporuri]

Useful information regarding the move to PySide6 - https://www.qt.io/blog/qt-for-python-6-released, https://doc.qt.io/qtforpython/porting_from2.html and https://doc.qt.io/qt-6/portingguide.html.

[rahulporuri]

Note that a PySide6 version 6.2 is planned for September 2021 which will be an LTS release. We can potentially wait for the LTS release and then add support.

[corranwebster]

PySide6 is now fairly well supported. PyQt6 still needs work, but we have another issue to track that.