Add kernel.py file containing kernel-related routines

[martinRenou]

We need those changes for xeus_python. We implemented widgets support, but it enforces us to mock IPython an ipykernel at different places: IPython.display.display, IPython.display.clear_output, IPython.get_ipython, ipykernel.comm.Comm. See https://github.com/QuantStack/xeus-python/blob/master/src/xinterpreter.cpp#L41.
Those changes help by putting all those imports in one place: kernel.py. So that we just need to mock ipywidgets.kernel in xeus_python.

[martinRenou]

I'm not sure about the changes on the __init__.py, I didn't know what to keep, what to change, and what to remove.

[martinRenou]

After in-person discussion with Sylvain, this doesn't seem to be useful. Maybe @jasongrout you have an idea? Do you think this can be removed? Or stay as an alias for register_comm_target for backward compatibility?

[maartenbreddels]

I like the title and comments and the description of the PR, and I'd really like to understand the issue, and the solution. Maybe a video call can help, or some more context. Happy to help with the PR though!

[martinRenou]

@maartenbreddels I edited the the PR description :)

[maartenbreddels]

Sorry that it wasn't clearer at first.

No worries. Starting to understand it better.

I didn't understand it was for xeus_python, now it starts to make more sense. I do think the get_ipython and get_kernel is a bit confusing. If there is a good reason to do I think some comments may help.

[martinRenou]

The thing is that get_ipython() is used at different places in the code, and not only to retrieve get_ipython().kernel, so your solution get_kernel = lambda: get_ipython().kernel does not really work. Also I changed the name because I would like, in xeus_python, to mock ipywidgets.kernel.get_kernel and not ipywidgets.kernel.get_ipython. Because at the end it's not ipython anymore

[martinRenou]

I think we can close this PR. It is a bit outdated. And IMO we should not mock ipywidgets to have it working in xeus-python, as in ipywidgets should not be a special case. I made a PR using IPython in xeus-python: jupyter-xeus/xeus-python#400, which should fix most of these issues.

We still need to mock ipykernel.comm though, as it is used by multiple libraries for using comms. We should find a nicer way to do this, but for now, mocking is our only solution.