Move this package upstream to Sphinx #146

As discussed at infrastructure tag-up today (2022-04-07) between @saimn , @eteq , @bsipocz , @tomdonaldson , and myself, this package might get better maintenance if it is in Sphinx proper. There is no record I can find but was told a previous such discussion with Sphinx was not successful but since that was a long time ago, maybe it is time to ask again?

Irrelevant if we go with #147