silx.gui.plot.PlotWidget: Fixed support of PySide6 #1555

Job	Run time
Python 3.8 on ubuntu-20.04 PyQt5 sdist	6m 12s
Python 3.8 on ubuntu-20.04 PyQt5 sdist	6m 12s
Python 3.10 on macos-latest PyQt5 wheel	8m 6s
Python 3.10 on macos-latest PyQt5 wheel	14m 15s
Python 3.8 on ubuntu-latest PySide6 sdist	8m 1s
Python 3.8 on ubuntu-latest PySide6 sdist	8m 20s
Python 3.9 on macos-latest PySide6 wheel	7m 42s
Python 3.9 on macos-latest PySide6 wheel	16m 3s
Python 3.11 on ubuntu-latest PyQt6 wheel	5m 56s
Python 3.11 on ubuntu-latest PyQt6 wheel	5m 56s
Python 3.11 on macos-latest PyQt6 wheel	4m 57s
Python 3.11 on macos-latest PyQt6 wheel	4m 57s
Python 3.9 on windows-latest No GUI wheel	5m 30s
Python 3.9 on windows-latest No GUI wheel	5m 30s
	1h 47m 37s

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