python3Packages.ray: fix `optional-dependencies.all` eval

Without the change the eval fails as:

    nix-repl> python3Packages.ray.optional-dependencies.all
    error:
       … while evaluating the attribute 'ray.optional-dependencies.all'
         at pkgs/development/python-modules/ray/default.nix:145:5:
          144|     air = lib.unique (data ++ serve ++ tune ++ train);
          145|     all = lib.flatten (builtins.attrValues optional-dependencies);
             |     ^
          146|     cgraph = [

       … while calling the 'concatMap' builtin
         at nixpkgs-master/lib/lists.nix:437:33:
          436|   */
          437|   flatten = x: if isList x then concatMap (y: flatten y) x else [ x ];
             |                                 ^
          438|

       (stack trace truncated; use '--show-trace' to show the full, detailed trace)

0 files changed, 0 insertions, 0 deletions