`ipykernel 6.5.0` is not compatible with `traitlets 4`

[trungleduc]

The latest release of ipykernel does not support traitlets 4 anymore, so it breaks our traitlets 4 test on CI. A short-term solution is to pin ipykernel to 5.5.5 in the test of traitlets 4 or python 3.6. For the long term, I think we can remove tests for these two configurations (#1027)