python3 fixes

examples/Arrow.py

examples/CLIexample.py

examples/ColorButton.py

examples/ConsoleWidget.py

examples/DataSlicing.py

examples/DataTreeWidget.py

examples/Draw.py

examples/ErrorBarItem.py

examples/Flowchart.py

examples/FlowchartCustomNode.py

examples/GLImageItem.py

examples/GLIsosurface.py

examples/GLLinePlotItem.py

examples/GLMeshItem.py

examples/GLScatterPlotItem.py

examples/GLSurfacePlot.py

examples/GLViewWidget.py

examples/GLVolumeItem.py

examples/GLshaders.py

examples/GradientWidget.py