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/development/python-modules/pplpy/default.nix

@@ -56,7 +56,7 @@ buildPythonPackage rec {
   meta = with lib; {
     description = "A Python wrapper for ppl";
     homepage = "https://gitlab.com/videlec/pplpy";
-    maintainers = with maintainers; [ timokau ];
+    maintainers = teams.sage.members;
     license = licenses.gpl3;
   };
 }