Try this:

pip install --user --ignore-installed cython

Then make sure that the Python user installs bin directory is on your path:

export PATH=$HOME/.local/bin:$PATH

Reload the path to make sure you're finding the right Cython:

hash -r

Now check you have the right Cython (in ~/.local/bin):

which cython

That. fixed it. Thanks, and thanks to all who helped me get scikit imstalled.