Contain usage of nest-asyncio

[SylvainCorlay]

[kevin-bates]

Hi @SylvainCorlay. The changes look good to me so far. Prior to merge, could you please add a description that includes the reason for these changes and what impact they might have on existing applications that subclass or use KernelManager directly? I suspect they too would need to move to BlockingKernelManager. Given this will likely be slated for 7.x (correct?), I don't see that being an issue.

[blink1073]

nest-asyncio usage was removed in #835