Don't import jupyter_client unless we're executing the notebook

[takluyver]

Closes gh-495

Also added jupyter_client as an explicit dependency for the tests, since it's imported in test code.

[minrk]

👍

[mpacer]

Why does setting the default value as a string work when previously it was being passed a class object?

Happy to merge, just wanted to know.

[takluyver]

The Type and Instance traitlets have some special functionality that you can supply classes as a string containing the full importable name of the class. They import it and get the actual class object when the object they're attached to is instantiated. Here's the code in Type:

https://github.com/ipython/traitlets/blob/4.3.1/traitlets/traitlets.py#L1601

[mpacer]

So, based on conda-forge/nbconvert-feedstock@1fce020 it suggests we should not have jupyter_client as any kind of dependency (strictly for tests or optionally)… I'm not sure how to fix that though while maintaining all current functionality.

[minrk]

I believe everything is correct now. It's a dependency for a feature, which is expressed via the setuptools extras_require field.