docs: update requirements to install six module

docs: update requirements to install six module

On the update of Sphinx version to 2.4.4, the "six" library won't be
installed automatically. (which is required by kfigure.py)

Main reason of this issue were occurred by the requirements changed from
the sphinx library. In Sphinx v1.7.9, six was listed on the
install_requires, but it has been removed since 2.x

The kfigure.py uses six library explicitly, adding six to
requirements.txt seems reasonable

Signed-off-by: JaeSang Yoo <jsyoo5b@gmail.com>
Reviewed-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/20201208014628.GA1361@JSYoo5B-Base.localdomain
Signed-off-by: Jonathan Corbet <corbet@lwn.net>