Replaced execute preprocessor internals with nbclient calls

[MSeal]

Finishes #821. Took me a while to get around to it, but here's the PR with all the stuff moved into nbclient deleted.

I found that given how light the wrapper is now only a few very basic tests should be present. I didn't like how I had to subprocess NBClient's class to account for all the traitlet assignment junk, but it works and leave the interface backwards compatible. If nbclient ever needs to move it's interface forward or off trailets or whatever, we can capture it here for translation against future versions.

[MSeal]

I think after this merges we can do 6.0.0a1 release

[choldgraf]

The amount red in this PR makes me happy

[MSeal]

Not sure why 3.5 is hanging on allow-errors -- will debug that later

[davidbrochart]

I tested this PR locally with python3.5, it seems to randomly hang on test_allow_errors (it doesn't happen with python3.6).
Asyncio was still missing some important features in 3.5, and nbclient is implemented using asyncio. We work around the limitations of 3.5 with e.g. the async_generator library, but there might be other things that are problematic.
Is there a plan to drop python3.5 in nbconvert, nbclient, and in the Jupyter ecosystem in general?
In Voila, switching to async means dropping python3.5 anyway, since we need async generators in Jinja, which is not going to happen in python3.5: pallets/jinja#1058

[choldgraf]

I am +1 on deprecating 3.5 after a deprecation cycle, now that we are already nearing 3.9 (obv the broader jupyter ecosystem will need to have a community decision on that)

[MSeal]

Yeah the general guidance I've been following is to support 3.5 until it cycles out of the supported path for python proper. The reason I've tried to keep maintaining it is there's some larger orgs whose internal default python version is still 3.5. Dropping support for it before python does would disrupt updates for those orgs. Still I think later this year 3.5 can drop. For now I'd be ok with marking those tests as 3.6+ only and putting a big warning in the release notes that 3.5 may have stability issues with this change given async support. Thoughts?

[davidbrochart]

Sounds good to me.

[MSeal]

Ok based on conversations across a few companies and in jupyter/nbclient#34 I'm going to update this PR and drop support for Python 3.5. The async code in upstream libraries has caused issues in each repo for python 3.5.

[MSeal]

Going to merge this to get some things moving