gnu: lightdm: Fix build.

This is a follow-up to commit 26876b6600,
migrate the package from Python 2 to Python 3 (but left some commented inputs
for tests, breaking the build).

* gnu/packages/display-managers.scm (lightdm)[native-inputs]: Un-comment
python-wrapper and python-pygobject.

gnu/packages/display-managers.scm

@@ -323,8 +323,8 @@ (define-public lightdm
              vala                       ;for Vala bindings
              ;; For tests
              dbus
-             ;; python-wrapper
-             ;; python-pygobject
+             python-wrapper
+             python-pygobject
              which
              yelp-tools))
       ;; Required by liblightdm-gobject-1.pc.