ikiwiki: Fix docutils support

Without this, the rst plugin complains as follows:

    cannot import docutils.core

Also, python3 is needed for the rst plugin to work.

pkgs/applications/misc/ikiwiki/default.nix

@@ -69,6 +69,7 @@ stdenv.mkDerivation {
       ${lib.optionalString cvsSupport ''--prefix PATH : ${cvsps}/bin ''} \
       ${lib.optionalString subversionSupport ''--prefix PATH : ${subversion.out}/bin ''} \
       ${lib.optionalString mercurialSupport ''--prefix PATH : ${mercurial}/bin ''} \
+      ${lib.optionalString docutilsSupport ''--prefix PYTHONPATH : "$(toPythonPath ${docutils})" ''} \
       ${lib.concatMapStrings (x: "--prefix PATH : ${x}/bin ") extraUtils}
     done
   '';

pkgs/top-level/all-packages.nix

@@ -21310,6 +21310,7 @@ in
   ike = callPackage ../applications/networking/ike { };
 
   ikiwiki = callPackage ../applications/misc/ikiwiki {
+    python = python3;
     inherit (perlPackages.override { pkgs = pkgs // { imagemagick = imagemagickBig;}; }) PerlMagick;
   };