docs: parser_yaml.py: fix backward compatibility with old docutils

As reported by Akira, older docutils versions are not compatible
with the way some Sphinx versions send tab_width. Add a code to
address it.

Reported-by: Akira Yokosawa <akiyks@gmail.com>
Closes: https://lore.kernel.org/linux-doc/598b2cb7-2fd7-4388-96ba-2ddf0ab55d2a@gmail.com/
Tested-by: Akira Yokosawa <akiyks@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>