sphinx: Pin to docutils-0.16

In newer docutils writers/latex2e/docutils.sty has a unicode character
in the comments which can in turn cause Python 3.6 at least to fail.
Pin to the previous docutils release for now.

Cc: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Tom Rini <trini@konsulko.com>