python36.withPackages fails, python35.withPackages succeeds?

I’m trying to go through the python nixpkgs manual line by line, and work the examples out to see if I can get something to work. When I paste the line

 nix-shell -p 'python35.withPackages(ps: with ps; [ numpy toolz ])'

it works and starts off to work. When I change the python35 to python36 it fails:

nix-shell -p 'python36.withPackages(ps: with ps; [ numpy toolz ])'
error: attribute 'withPackages' missing, at (string):1:94
(use '--show-trace' to show detailed location information)

What’s the new standard if .withPackages is being removed?

Nevermind – I had

self: super: {
  python36.pkgs.rsa = super.python36.pkgs.rsa.override({LC_ALL="en_US.utf8";});

which was an attempt to set an environment variable in an overlay. Instead it just causes Byzantine failure. Guess overlays are too tricky for me to use.