maintainers: create sage team

Maintaining all the packages whose updates might break some aspect of
sage.

For reference:
https://github.com/NixOS/nixpkgs/pull/103810#issuecomment-727536510

pkgs/data/misc/graphs/default.nix

@@ -20,6 +20,6 @@ stdenv.mkDerivation rec {
     description = "A database of graphs";
     license = licenses.gpl2;
     platforms = platforms.all;
-    maintainers = with maintainers; [ timokau ];
+    maintainers = teams.sage.members;
   };
 }