Update metadata_display plugin for Python 3.

Switching from iteritems() to items() is technically less memory efficient on
Python 2, but this appeared be a significantly cleaner approach than adding six
to the template context. It's also the right choice assuming the project will at
some point go Python 3 only.

mediagoblin/plugins/metadata_display/templates/mediagoblin/plugins/metadata_display/metadata_table.html

@@ -23,7 +23,7 @@
   {#- NOTE: In some smart future where the context is more extensible,
         we will need to add to the prefix here-#}
   <table class="metadata_info">
-    {%- for key, value in metadata.iteritems() if not key=='@context' %}
+    {%- for key, value in metadata.items() if key != '@context' %}
       {% if value -%}
         <tr>
           <th>{{ rdfa_to_readable(key) }}</th>