Fix opensearch template.

Doc/tools/sphinxext/opensearch.xml

-<?xml version="1.0" encoding="UTF-8"?>
-<OpenSearchDescription xmlns="http://a9.com/-/spec/opensearch/1.1/">
-  <ShortName>Python Docs</ShortName>
-  <LongName>Python Documentation</LongName>
-  <Description>Search the Python documentation</Description>
-  <InputEncoding>utf-8</InputEncoding>
-  <Url type="text/html" method="get" template="{{ pathto('search') }}?">
-    <Param name="q" value="{searchTerms}" />
-    <Param name="check_keywords" value="yes" />
-    <Param name="area" value="default" />
-  </Url>
-  <Image height="16" width="16" type="image/x-icon">http://www.python.org/images/favicon16x16.ico</Image>
-</OpenSearchDescription>
-
+{% extends "!opensearch.xml" %}
+{% block extra -%}
+<Image height="16" width="16" type="image/x-icon">http://www.python.org/images/favicon16x16.ico</Image>
+{%- endblock %}