Python 3.6 EOL

So Python 3.6 is EOL this month. We still notionally support 3.4 (though even 3.5 isn’t actively tested), so is it OK to change this:

set(vtk_python_min_version "3.4")
set(vtk_python_version_support "3.6")

to:

set(vtk_python_min_version "3.4")
set(vtk_python_version_support "3.7")

where min_version is what is required, but below version_support, VTK’s configure makes noise unless VTK_LEGACY_SILENT. I’ll note that VTK_LEGACY_REMOVE raises min_version to version_support if set.

Thoughts?