vtk test should use python3
dune/grid/io/file/test/checkvtkfile.hh currently uses
python, it should prefer
python3 and only use
python(2) if there is no vtk module available for python3.
The python-vtk6 module was just removed from Debian, but older release we still support in DUNE don't have python3-vtk7. So supporting only Py3 doesn't work (I did that in the Debian package for now where this is okay).