Mention jupyter server extension in the tests docs

[jtpio]

Fixes #220.

The reason why the tests mentioned in #220 were failing when running locally is because they required the Jupyter server extension to be enabled. Enabled with the jupyter extension command (not jupyter serverextension).

This change adds this information to the contributing guide.

[maartenbreddels]

But how can it run on travis then? I don't see we execute this in travis.yml. Any idea?

[jtpio]

I think pip install with without the -e picks up the server extensions automatically because the data files will end up at the right place.

[jtpio]

And travis does: pip install ".[test]"

[jtpio]

cc @martinRenou if you want to give it a try.

[maartenbreddels]

That might mean that some of the files in share are not added to data_files in setup.py? I would expect the opposite behaviour actually.

[SylvainCorlay]

I think pip install with the -e picks up the server extensions automatically because the data files will end up at the right place.

I don't think data files are installed at all with -e.

[jtpio]

pip install with the -e

I meant without (sorry for the confusion).

[martinRenou]

Yep, that fixed it for me :) (tried with a newly created environment)

[maartenbreddels]

Ok, that makes more sense.

[maartenbreddels]

We could also have a custom develop cmd, like we do here:
https://github.com/mariobuikhuizen/voila-vuetify/blob/master/setup.py#L50

setup.py will be a monster though.

[jtpio]

Yes it would be great if the develop command could take care of that.

We can create a follow-up issue and see if it makes sense to add it to the setup.py later on.

[jtpio]

Will be merging this one for now, since it solves the original issue of being able to run the tests locally.