In Python 3.8, the reserved "tp_print" slot was changed from a function
pointer to a number, which broke the Python wrapping code in vtk@8
(causing "cannot convert 'std::nullptr_t' to 'Py_ssize_t'" errors in
various places). This is fixed in vtk@9.0.0.
This patch:
1) adds vtk@9.0.0
2) updates depends_on constraints to only use python@3.8: for vtk@9:
vtk@:8 depends on python@2, and vtk@8.0.1:8.9.9 depends on python@:3.7
3) Adds CMake flag VTK_PYTHON_VERSION=3 when using python@3 with vtk@9
4a948060cf