ipywidgets expects Comm, not BaseComm #1090

Hi! I've been trying to track down an issue with my usage of ipywidgets, described on discourse. The basic gist is:

• I'm attempting to create a widget from the client by using a model.widget_manager's method new_widget. In previous versions, this worked, but now, it doesn't.
• The issue seems to stem from the kernel attempting to create a new comm instance for a widget, but supplying to that widget a BaseComm rather than a Comm, which throws a validation error.

After digging through this, it looks to me like ipywidgets declares comm = Instance('ipykernel.comm.Comm', allow_none=True), but ipykernel.ipkernel.create_comm explicitly creates a BaseComm, so the traitlet doesn't validate, and the comm fails to properly initialize. This results in the widgets not being synced back to the kernel from the frontend.

I confess that while I've spent time going through the code, I'm not entirely sure if this could potentially be a version mismatch, or if this is an oversight for a use case that may not come up terribly frequently.

My versions:

• ipywidgets - 8.0.4
• ipykernel - 6.21.1
• comm - 0.1.2

I was able to address the problem I was having by changing the import and usage of BaseComm to Comm in ipkernel.py, and so if this is an acceptable solution I'd be happy to issue a pull request. (My hesitance is that there may be unexpected downstream effects of a change to something so deep in the kernel code, and so I wanted to open the issue as possible first step.)