Make ThreadsafeMatplotlibAxes work with pyside2

[danielballan]

It looks like pyside is stricter than PyQt5 about adding attributes to a subclass on QObject.

_____________________________ test_image[QtFigure] _____________________________

FigureView = <class 'bluesky_widgets.qt.figures.QtFigure'>

    def test_image(FigureView):
        "Test Images with a 2D array."
        run = build_simple_run({"ccd": numpy.random.random((11, 13))})
        model = Images("ccd")
>       view = FigureView(model.figure)

bluesky_widgets/models/_tests/test_image.py:12: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
bluesky_widgets/qt/figures.py:165: in __init__
    model=axes_spec, axes=axes
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = <bluesky_widgets.qt.figures.ThreadsafeMatplotlibAxes(0x55f4636aedf0) at 0x7f49ac055280>
args = ()
kwargs = {'axes': <AxesSubplot:>, 'model': AxesSpec(lines=LineSpecList([]), images=ImageSpecList([]), title=None,x_label=None, y_label=None, aspect=None, x_limits=None, y_limits=None, uuid=UUID('0f121abd-8139-40a6-a34c-b749ae00d352'))}

    def __init__(self, *args, **kwargs):
>       super().__init__(*args, **kwargs)
E       AttributeError: 'model()' is not a Qt property or a signal

bluesky_widgets/qt/figures.py:39: AttributeError