[3.14] gh-135755: Document __future__.* and CO_* as proper Sphinx objects (GH-135980) (GH-136370)

* Turn the __future__ table to list-table.
  This'll make it easier to add entries that need longer markup
* Semantic markup for __future__ feature descriptions.
* Document CO_* C macros.

(cherry picked from commit 2468aafe984fdf923811ef0c6969e3d6c1b92a82)

Co-authored-by: Petr Viktorin <encukou@gmail.com>