ein:jupyter-crib-token: check that the process is actually jupyter (#867)

[sam-s]

This will avoid the persistent 401 unauthorized error when there is a process with the same PID as an older jupyter.

[dickmao]

In the tradition of great customer service, I will wait until another customer complains about the same problem before escalating.

[sam-s]

this is a rare condition by it costs you nothing to accept PR!