The use of nest_asyncio makes nbconvert --to webpdf hang when used with --execute

[SylvainCorlay]

This comes with an issue in pyppeteer: pyppeteer/pyppeteer#99

How could we avoid using nest_asyncio for nbconvert --execute?

[davidbrochart]

Instead of using execute() in nbconvert, we could use async_execute() and make it run until complete ourselves.
The problem is that execute() calls run_sync(), which creates an event loop the first time, and next time uses nest_asyncio, but it doesn't have to be (we could just get the current event loop). I think we should have a global option to use nest_asyncio or not.

[davidbrochart]

I think this fixes the issue: jupyter/nbconvert#1406

[SylvainCorlay]

I reported the issue in nest-asyncio and it seems that they fixed it right away: erdewit/nest_asyncio#34

If they make a patch release including this, we should require it.