jupyter command ends up installed with each Python version

[takluyver]

We tell people to install ipykernel on the 'other' version of Python to make the Python kernel available for that Python. But that depends on jupyter_core, so the jupyter command gets installed again with its shebang pointing to the other version of Python, the one on which they have no Jupyter applications installed.

I'm not sure if this actually has any adverse effects, but it seems like a potential source of confusion, e.g. if jupyter_core is updated on one Python but not the other, or just using jupyter troubleshoot to troubleshoot.

From ipython/ipykernel#40