docs: sphinx-extensions: make rstFlatTable work with docutils 0.13

In docutils 0.13, the return type of get_column_widths method of the
Table directive has changed [1], which breaks our flat-table directive
and leads to a TypeError when trying to build the docs [2].

This patch adds support for the new return type, while keeping support
for older docutils versions too.

[1] https://sourceforge.net/p/docutils/patches/120/
[2] https://sourceforge.net/p/docutils/bugs/303/

Signed-off-by: Dmitry Shachnev <mitya57@debian.org>
Cc: <stable@vger.kernel.org> # 4.8.x-
Signed-off-by: Jonathan Corbet <corbet@lwn.net>