Tests failing due to regression in ipynb error messages

[choldgraf]

Describe the problem

The tests are currently failing because the output syntax for errors seem to have changed:

https://github.com/executablebooks/MyST-NB/runs/3210843582#step:5:233

I tried debugging this locally but it seems like the ipykernel_NNN part of the error message changes each time the test is run, so always fails.

@chrisjsewell suggested the following may be the issue:

The ipykernel now being installed is ipykernel-6.0.3, whereas previously it was ipykernel-5.5.5, so the easiest thing would be to just add ipykernel~=5.5 to the testing extra