Remove compatibility with old recommonmark versions

Remove compatibility with old recommonmark versions

It's hard trying to keep compatibility with both very old (<1.8),
old (1.8) and modern (Sphinx), while also supporting old recommonmark;
recommonmark 0.5 was released in January 2019.

So let's drop the old support and just keep support for Sphinx 1.8 and
3.0+, and more modern recommonmark.