Upgrade Sphinx to 1.7.1

[fabaff]

Description

Changelog: http://www.sphinx-doc.org/en/stable/changes.html

Checklist:

If the code communicates with devices, web services, or third-party tools:

• New dependencies have been added to requirements_all.txt by running script/gen_requirements_all.py.

[balloob]

Last time the sphinx build was broken for months. Did you build a local sphinx build ?

[fabaff]

Did you build a local sphinx build ?

Of course. There are usually a couple of warning but nothing fatal. Not sure why the move of the constants in the last November that caused the build issue wasn't detected earlier. I suspect that I didn't clean the output directory on a regular base.