ENH: Support for Python3

[jcfr]

Updated code is expected to work with both Python2 and Python3

Slicer migration guide describes how to update the code. See https://www.slicer.org/wiki/Documentation/Nightly/Developers/Tutorials/MigrationGuide/SlicerExtension#Slicer_5.0:_Python2_to_Python3

This commit was crafted by:

• installing future

  Slicer_DIR=/path/to/Slicer-SuperBuild/Slicer-build
  ${Slicer_DIR}/../python-install/bin/PythonSlicer -m pip install
future

• applying all fixes

  for f in `find ./ -name "*.py"`; do \
    ${Slicer_DIR}/../python-install/bin/PythonSlicer \
      --launch futurize -w $f; \
  done

• reviewing the changes and updating as needed.