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

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>
Gbp-Pq: Topic bugfix/all
Gbp-Pq: Name docs-sphinx-extensions-make-rstFlatTable-work-with-d.patch