Another note (for some this is obvious, but I’m sure many noobs like me with find this helpful):
I had to also set PYTHONPATH to make ‘import vtk’ work: ‘sudo make install’ puts the package in ‘/usr/local/lib/python3.5/site-packages’, which is not in the search path for imports. Under ubuntu (and other distributions derived from debian), we find instead ‘/usr/local/lib/python3.5/dist-packages’, which is the destination for packages installed by pip(3). I’ve done this by adding this line to /etc/environment:
PYTHONPATH="/usr/local/lib/python3.5/site-packages"
(I didn’t come up with this solution myself, I found it online, but unfortunately I haven’t saved the links)