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>

Documentation/sphinx/rstFlatTable.py

@@ -157,6 +157,11 @@ class ListTableBuilder(object):
     def buildTableNode(self):
 
         colwidths    = self.directive.get_column_widths(self.max_cols)
+        if isinstance(colwidths, tuple):
+            # Since docutils 0.13, get_column_widths returns a (widths,
+            # colwidths) tuple, where widths is a string (i.e. 'auto').
+            # See https://sourceforge.net/p/docutils/patches/120/.
+            colwidths = colwidths[1]
         stub_columns = self.directive.options.get('stub-columns', 0)
         header_rows  = self.directive.options.get('header-rows', 0)