Build both python2 and python3 binding

This issue was created automatically from an original Mantis Issue. Further discussion may take place here.

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It would be useful to have both python 2 and python3 bindings built at the same time in the cmake if both version are present. See this downstream feature request (also by myself) for example.

https://bugs.archlinux.org/task/48113?string=vtk&project=5&search_name=vtk&type%5B0%5D=&sev%5B0%5D=&pri%5B0%5D=&due%5B0%5D=&reported%5B0%5D=&cat%5B0%5D=&status%5B0%5D=open&percent%5B0%5D=&opened=&dev=&closed=&duedatefrom=&duedateto=&changedfrom=&changedto=&openedfrom=&openedto=&closedfrom=&closedto=