PySide6 compatibility

[chrisjbillington]

Add PySide6 compatibility, whilst still being compatible with PyQt5 by virtue of requiring qtutils>=4.0, which provides bidirectional compatibility aliases.

[dihm]

During testing, I also get the following deprecation notice

C:\Users\naqsL\src\labscript-suite\runmanager\runmanager\__main__.py:188: DeprecationWarning: 
Function: 'QMouseEvent.pos() const' is marked as deprecated, please check the documentation for more information.
  index = self.indexAtPos(event.pos())

It appears PySide6 deprecated in 6.0 and wants QMouseEvent.position() instead (which is not available in PyQt5. Probably going to just leave it for now so we can support both libraries without too much customization.