python.pkgs.roboschool: mark as removed

I recently removed roboschool, but did not add an alias with a removal
notice. As suggested by [1].

[1] https://github.com/NixOS/nixpkgs/pull/154998#issuecomment-1013334113

pkgs/top-level/python-aliases.nix

@@ -89,6 +89,7 @@ mapAliases ({
   rdflib-jsonld = throw "rdflib-jsonld is not compatible with rdflib 6"; # added 2021-11-05
   repeated_test = throw "repeated_test is no longer maintained"; # added 2022-01-11
   requests_toolbelt = requests-toolbelt; # added 2017-09-26
+  roboschool = throw "roboschool is deprecated in favor of PyBullet and has been removed"; # added 2022-01-15
   rotate-backups = throw "rotate-backups was removed in favor of the top-level rotate-backups"; # added 2021-07-01
   ruamel_base = ruamel-base; # added 2021-11-01
   ruamel_yaml = ruamel-yaml; # added 2021-11-01