bug#27572: simultaneous installation of python2 and python3 fails

[Diego Nicola Barbato]

When running the command "guix pull && guix package -i address@hidden python"
instead of installing the latest version of python2 and python3, as it
used to do until about four weeks ago, guix gives following error
message (hash values replaced with ...):

  guix package: error: profile contains conflicting entries for python:out
  guix package: error:   first entry: address@hidden:out 
/gnu/store/...-python-2.7.13
  guix package: error:   second entry: address@hidden:out 
/gnu/store/...-python-3.5.3

This error could be reproduced in a VM created with guix system vm-image
and following config file:

  (use-modules (gnu) (gnu packages) (gnu system nss))
  (use-service-modules desktop xorg)
  (use-package-modules certs gnome xdisorg lisp)

  (operating-system
   (host-name "Test")
   (timezone "Europe/Zurich")
   (locale "de_CH.UTF-8")

   (bootloader (grub-configuration (device "/dev/sda")
                                   (timeout 1)))

   (file-systems (cons (file-system
                        (mount-point "/")
                        (device "/dev/sda1")
                        (type "ext4"))
                       %base-file-systems))

   (users (cons (user-account
                 (name "user1")
                 (comment "User")
                 (group "users")
                 (supplementary-groups '("wheel" "audio" "video"))
                 (home-directory "/home/user1"))
                %base-user-accounts))

   (packages (cons* nss-certs
                    sbcl-stumpwm (list sbcl-stumpwm "lib")
                    %base-packages))

   (services (cons* (screen-locker-service xscreensaver "xscreensaver")
                    (console-keymap-service "de_CH-latin1")
                    %desktop-services))

   (name-service-switch %mdns-host-lookup-nss))