Run tests for Python 3 and Plone 6

[pbauer]

Extend the github actions matrix to also test Python 3 and Plone 6
The robot-tests are still broken though...

[wesleybl]

I think that before this is done, a Pull Request from the plone52 branch should be opened. There is already a lot of change there.

@pbauer @cleberjsantos , do you plan to open PR?

[cleberjsantos]

@wesleybl Sounds good

[pbauer]

The extended matrix should be added to the branch of #900