Fix call to ScaleEngine.autoScale

The original `QwtScaleEngine.autoScale` mutated the passed values in-place, but this was missed when the code was translated to Python, where `autoScale` returns the new values as a tuple.

qwt/plot.py

@@ -954,7 +954,7 @@ def updateAxes(self):
                 d.isValid = False
                 minValue = intv_i.minValue()
                 maxValue = intv_i.maxValue()
-                d.scaleEngine.autoScale(d.maxMajor, minValue, maxValue, stepSize)
+                minValue, maxValue, stepSize = d.scaleEngine.autoScale(d.maxMajor, minValue, maxValue, stepSize)
             if not d.isValid:
                 d.scaleDiv = d.scaleEngine.divideScale(
                     minValue, maxValue, d.maxMajor, d.maxMinor, stepSize