Make splash screen widget compatible with PySide6

[chrisjbillington]

Unsure about the status of the other widgets in labscript-utils, figure I'll address them when testing the other applications with PySide6. The need to modify the splash screen became apparent when adding PySide6 compatibility for runmanager (labscript-suite/runmanager#118).

This now works with either PyQt5 or PySide6, and is a bit of an improvement regardless - it follows more closely what Qt's build-in spash screen widget does.

[chrisjbillington]

FWIW I don't have a general plan to port everything to PySide6 imminently, rather I'm porting an unrelated application and was using runmanager as a bit of a testbed. But qtutils 4.0 supports PySide6 now so porting labscript suite applications is now possible.

With PySide6 being a sensibly-behaving library (compared to PySide2 which had some major deficiencies) and the licensing situation favouring it over PyQt, it doesn't seem like we should go out of our way to support PyQt6 without good reason. Anaconda is still PyQt5-only, and who knows what they'll do - whether they'll include PySide6 or PyQt6 when they drop PyQt5. Maybe conda (or at least Anaconda and their main channel) will cease to be relevant by then and make our lives easier.