Evaluate whether we should/could replace nbval with nbclient

[ceball]

I recently noticed that notebook execution has been extracted from nbconvert into a standalone library, nbclient (https://github.com/jupyter/nbclient/). Should evaluate whether:

• We can replace import/usage of nbval with nbclient
• We can replace internal partial copy/paste of nbval with nbclient
• (nbval itself could use parts of nbclient?)

The motivation being that nbval has more customized kernel communication requirements than nbcelltests (nbval is inspecting outputs etc), which means:

• nbval doesn't currently provide a convenient method we can use to communicate with the kernel that does not also involve dealing with outputs, so we have a copy/paste/modify of an nbval method in celltests
• nbval might introduce issues (e.g. Certain cell failures are not detected #190)

Is nbclient simpler/safer/more likely to stay up to date? Does it provide an interface we can use? Etc