Python 3.8 support

Fedora Rawhide is moving to python 3.8. vtk 8.2.0 fails to compile with:

/Wrapping/PythonCore/PyVTKMethodDescriptor.cxx:230:1: error: cannot convert 'std::nullptr_t' to 'Py_ssize_t' {aka 'long int'} in initialization
BUILDSTDERR:   230 | };
BUILDSTDERR:       | ^
BUILDSTDERR: make[2]: *** [Wrapping/PythonCore/CMakeFiles/vtkWrappingPythonCore.dir/build.make:144: Wrapping/PythonCore/CMakeFiles/vtkWrappingPythonCore.dir/PyVTKMethodDescriptor.cxx.o] Error 1

I python 3.8 support in master yet? Is there a straightforward patch to apply, or are we waiting for 8.3? Thanks!