Sphinx 2.0 switched the default HTML builder from HTML4 to HTML5 [1].
Since some of our tests are validating raw HTML output, this has caused
them to break. We can't drop support for Sphinx < 2.0 yet since Sphinx
2.0 doesn't support Python 2.7, so simply if-else this mofo until such a
time as we _can_ drop support.
[1] https://github.com/sphinx-doc/sphinx/issues/4587
Change-Id: I4631169908e34fa924f5a0a783a94ad9c1215d9f
Signed-off-by: Stephen Finucane <stephenfin@redhat.com>