Fix running tests on CI

[jtpio]

As noticed in #217, CI seems to be failing.

This is likely because a dependency pulls a newer version of jupyterhub. For example dockerspawner does not put an upper limit on the jupyterhub dependency here: https://github.com/jupyterhub/dockerspawner/blob/955e64222447c22df6a708b7d63d90059303337d/requirements.txt#L3