I had the same problem.
How I fixed it:
pip3 install --upgrade Cython
sudo pip3 install --upgrade git+https://github.com/Xilinx/PYNQ-ComputerVision.git
Then you will probably run into this problem:
ImportError: libopencv_shape.so.3.2: cannot open shared object file: No such file or directory
Follow the solution from rock: ImportError: libopencv_shape.so.3.2: cannot open shared object file: No such file or directory
You have to manually install the libopencv packages (around 17) after that it should work
I used PYNQ 2.5 on an Ultra96-V2