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/applications/science/math/R/default.nix

@@ -113,6 +113,6 @@ stdenv.mkDerivation rec {
     platforms = platforms.all;
     hydraPlatforms = platforms.linux;
 
-    maintainers = with maintainers; [ peti timokau ];
+    maintainers = with maintainers; [ peti ] ++ teams.sage.members;
   };
 }