Drop support for JupyterHub 0.9 and Python 3.6

[consideRatio]

I think this project shouldn't attempt to support JupyterHub 0.9 all the way to JupyterHub 4. Its currently declaring 0.9 as a lower bound, and tests against 1.0.0 and hasn't yet started testing JupyterHub 4.

I suggest dropping support for 0.9 and 1.0.0, declaring it requires jupyterhub>=1.5.1 or jupyterhub>=2.3.1 which are the latest versions of JupyterHub 1 and 2.

I suggest dropping support for Python 3.6, as for example Python 3.7 soon has reached end of life as well.

[consideRatio]

Done via #273.