sage: fix test suite with docutils 0.15

[timokau]

Motivation for this change

The better way to fix this would be to backport the upstream sphinx
patch:

sphinx-doc/sphinx@faedcc4

Unfortunately it doesn't apply cleanly and isn't worth the effort
of backporting. Let's hope we can switch to python3 sage and the recent
sphinx version that comes with it before this becomes a problem.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[timokau]

Note that this will only fix half of #81449, with the other half being covered in #82239. I separated the two because #82239 touches a different package.