Notebook tests can fail

[opcode81]

See https://github.com/jambit/sensAI/runs/2884663556?check_suite_focus=true

RuntimeError: Kernel died before replying to kernel_info zmq.error.ZMQError: Address already in use

@MischaPanch, are you familiar with such errors? Could this be related to the newly introduced caching (#31)?

[MischaPanch]

I don't think it has anything to do with caching. Seems to be related to this long standing issue: https://github.com/jupyter/nbconvert/issues/1066

The developer said he might get around to fix it some time soon, maybe we can just wait and restart the failing pipelines when this happens for the moment? Doesn't seem to happen all too often

[schroedk]

I got the same error while running two pipelines in parallel. While the first build was sucessful, the second one failed with the error message above. Rerunning the second build (after the first one finished) was sucessful. I will try to reproduce this behavior.