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>

Documentation/sphinx/parser_yaml.py

@@ -77,6 +77,10 @@ class YamlParser(Parser):
 
                 result.append(line, document.current_source, lineoffset)
 
+            # Fix backward compatibility with docutils < 0.17.1
+            if "tab_width" not in vars(document.settings):
+                document.settings.tab_width = 8
+
             rst_parser = RSTParser()
             rst_parser.parse('\n'.join(result), document)