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

[t20100]

Checklist:

• [x] The PR title is formatted as: <Module or Topic>: <Action> <Summary> (see contributing guidelines)

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This PR adds an accepted exception type to those that can happen during PlotWidget.hideEvent with PySide6. This aims at fixing the CI on Windows with PySide6:

__________________ TestPixelIntensitiesHisto.testShowAndHide __________________
self = <silx.gui.plot.test.testPixelIntensityHistoAction.TestPixelIntensitiesHisto testMethod=testShowAndHide>
    def tearDown(self):
        del self.plotImage
>       super(TestPixelIntensitiesHisto, self).tearDown()
venv_test\Lib\site-packages\silx\gui\plot\test\testPixelIntensityHistoAction.py:50: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
self = <silx.gui.plot.test.testPixelIntensityHistoAction.TestPixelIntensitiesHisto testMethod=testShowAndHide>
    def tearDown(self):
        self.qapp.processEvents()

        if len(self.__class__._exceptions) > 0:
            messages = "\n".join(self.__class__._exceptions)
>           raise AssertionError("Exception occured in Qt thread:\n" + messages)
E           AssertionError: Exception occured in Qt thread:
E           Traceback (most recent call last):
E           AttributeError: Slot 'FitAction::' not found.
venv_test\Lib\site-packages\silx\gui\utils\testutils.py:193: AssertionError
------------------------------ Captured log call ------------------------------
ERROR    silx.gui.plot.PlotWidget:PlotWidget.py:835 Exception occured: The SignalInstance object was already deleted