Luckily we don't rely on SphinxDirective functionality which does not
exist in 1.6.7. Replace it with docutils Directive.

transform_content() callback was added only in Sphinx 3.0.0.
Detect if it was not called and call it manually.
The transform_content() function requires access to inner "contentnode"
which is created inside run(). This workaround relies on the order of
node as it was in the pre-3.0.0 versions, but it should not matter as
new versions will not trigger the workaround.