Build docs on python3.5 with linkcheck running on python 2.7

[jenshnielsen]

This is an alternative to #5338 which runs the yet not python 3.5 compatible linkchecker on python 2.7 but builds the docs with python 3.5

[jenshnielsen]

I have removed jsonshema and mistune they are not needed anymore. They were only there because ipython had a bud in its dependencies

[mdboom]

In terms of catching bugs in and triggered by the examples, would it make sense to build the docs in both Py3 and Py2 (in separate Travis configs so they run in parallel)?

[jenshnielsen]

I have thought about that and it probably would. It's of cause a tradeoff between the time spend on building the docs vs how much more coverage it gives. Perhaps we should only do that after #5597 is merged?

[mdboom]

Waiting for #5597 probably makes sense -- not that that requires an IPython patch (merged, but not released) and either a new release of numpydoc or moving to sphinx-napoleon (#5743)