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/pari-seadata-small/default.nix

@@ -21,6 +21,6 @@ stdenv.mkDerivation {
     homepage = "http://pari.math.u-bordeaux.fr/";
     license = licenses.gpl2Plus;
     platforms = platforms.all;
-    maintainers = with maintainers; [ timokau ];
+    maintainers = teams.sage.members;
   };
 }