Fix CI by adding ipykernel to requirements (Cherry-pick of #230) (#245

)

CI started failing with

> sphinx.errors.ExtensionError: Unable to find kernel (exception: No
such kernel named python3)

I used a text comparison between our requirements installed in CI from
last week vs this week. The main difference is `ipykernel` no longer
showing up in the list of reqs. That is due to the patch bump of
`ipywidgets` and its change here:

jupyter-widgets/ipywidgets@785d159#commitcomment-106272180

In the meantime to them possibly reverting that change, this fixes CI.

requirements-dev.txt

@@ -1,3 +1,4 @@
+ipykernel
 jupyter-sphinx
 numpy
 matplotlib