Upgrade Python version

[allywarner]

• Upgrade python to 3.6.7.
• Add a list of python versions to choose from. (3.4, 3.5, 3.6)

[dcwhite]

I'm going to try to incorporate parts of https://github.com/Slicer/Slicer/blob/master/SuperBuild/External_python.cmake and https://github.com/python-cmake-buildsystem/python-cmake-buildsystem