Add documentation build to tools/test_pr.py

[bfroehle]

It'd be pretty neat if we could also build the IPython documentation as part of tools/test_pr.py, as this would avoid issues like #2200.

Actually building the docs is pretty simple. The IPython debian package builds them by running:

PYTHONPATH=`pwd` make -C docs html

This only needs to happen for one version of Python, and only if Sphinx (and other requisite packages) are installed.

[minrk]

There are actually quite a lot of environments where the docs don't build, or at least there used to be.

[minrk]

Wait, I think I'm thinking of the PDF docs, which are quite finicky - the HTML docs are easy, nevermind.

[takluyver]

This should be an optional extra, though, because it slows down the PR testing even more (I actually want to speed it up, see #2026). And as an optional extra, it wouldn't get used very often.

Perhaps a better approach to this is to set up a ShiningPanda job that builds the docs. That should be quite easy, I'll have a go at it now.

[takluyver]

Starting its first build: https://jenkins.shiningpanda.com/ipython/job/ipython-docs/

[takluyver]

After a bit of fiddling, I've successfully replicated the error from #2200. When we commit the fix, hopefully the build will pass again.

[fperez]

I agree with @takluyver, test_pr if anything is already too slow. I prefer the CI approach in this case. @bfroehle, if you agree we can close this one and go with CI instead.

[bfroehle]

You bet.