aws_shell: move to pythonPackages

This allows to have awscli running both under python 2 and 3 as
necessary.

pkgs/top-level/all-packages.nix

···
 
   awslogs = callPackage ../tools/admin/awslogs { };
 
-  aws_shell = pythonPackages.callPackage ../tools/admin/aws_shell { };
+  aws_shell = pythonPackages.aws_shell;
 
   azure-cli = nodePackages.azure-cli;
 

pkgs/top-level/python-packages.nix

···
 
   awscli = callPackage ../tools/admin/awscli { };
 
+  aws_shell = callPackage ../tools/admin/aws_shell { };
+
   # packages defined elsewhere
 
   backports_csv = callPackage ../development/python-modules/backports_csv {};