Documentation/sphinx: fix kernel-doc extension on python3

Reconcile differences between python2 and python3 on dealing with
stdout, stderr from Popen. This fixes "name 'unicode' is not defined"
errors on python3. We'll need to try to keep the extension working on
both python-sphinx and python3-sphinx so we don't need two copies.

Reported-and-tested-by: Marius Vlad <marius.c.vlad@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>