vtk6 is marked for autoremoval from testing

[Debian testing autoremoval watch]

vtk6 6.3.0+dfsg1-10 is marked for autoremoval from testing on 2017-08-16

It (build-)depends on packages with these RC bugs:
867037: python-txaio: python-txaio FTBFS: test_chained_callback[asyncio] failed