BUG: Fix double image from py3 true division

This example was failing due to a mixture of float and double image.

The result of the true division operator on images in SimpleITK is a
double image. We import the true division as the standard operator,
then convert the result back to the expected.

Change-Id: Ie23c178657405944f7f013c0e9663f1b708a573f
Merge topic 'FixPy32FFTConvolutionExample' into next

cd90498 BUG: Fix double image from py3 true division