install-deps.sh: always use python3-sphinx to build the docs

install-deps.sh: always use python3-sphinx to build the docs

python-sphinx creates a symlink pointing ../share/sphinx/scripts/python2/sphinx-build
to /usr/bin/sphinx-build even if python3-sphinx is already installed.
in that case, if python-sphinx is installed after python3-sphinx,
sphinx-build is in python2. and it breaks the build of master, as we are
using python3 syntax in conf.py since
e9e17b9ceff0c862c8f29d629ad54a1dc401ed73.

and we are still using python2 as well as "python-sphinx" when building nautilus,
so if a build slave happen to compile nautilus before it is scheduled to
build master or a wip- branch, the doc build fails.

in this change, python-sphinx is uninstalled, if /usr/bin/sphinx-build
is found to be written in python2.

Signed-off-by: Kefu Chai <kchai@redhat.com>