Update template for newest Sphinx.
diff --git a/Doc/tools/sphinxext/indexcontent.html b/Doc/tools/sphinxext/indexcontent.html
index 218f346..67a9eaf 100644
--- a/Doc/tools/sphinxext/indexcontent.html
+++ b/Doc/tools/sphinxext/indexcontent.html
@@ -1,3 +1,5 @@
+{% extends "defindex.html" %}
+{% block tables %}
<p><strong>Parts of the documentation:</strong></p>
<table class="contentstable" align="center"><tr>
<td width="50%">
@@ -54,3 +56,4 @@
<p class="biglink"><a class="biglink" href="{{ pathto("copyright") }}">Copyright</a></p>
</td></tr>
</table>
+{% endblock %}