ikiwiki: Use python with pygments package

Pygments package is needed by docutils to support syntax
highlighting via the ".. code::" directive.

Without pygments, the rendered pages contain errors like this:

Cannot analyze code. Pygments package not found.

pkgs/applications/misc/ikiwiki/default.nix

···
       TimeDate gettext makeWrapper DBFile CGISession CGIFormBuilder LocaleGettext
       RpcXML XMLSimple ImageMagick YAML YAMLLibYAML HTMLTree AuthenPassphrase
       NetOpenIDConsumer LWPxParanoidAgent CryptSSLeay ])
-    ++ lib.optionals docutilsSupport [python docutils]
+    ++ lib.optionals docutilsSupport [
+         (python.withPackages (pp: with pp; [ pygments ]))
+         docutils
+       ]
     ++ lib.optionals gitSupport [git]
     ++ lib.optionals monotoneSupport [monotone]
     ++ lib.optionals bazaarSupport [breezy]