Add link to the favicon to the docs.

Doc/tools/sphinxext/layout.html

 {% extends "!layout.html" %}
 {% block rootrellink %}
-<li><img src="{{ pathto('_static/py.png', 1) }}" alt="" style="vertical-align: middle; margin-top: -1px"/></li><li><a href="{{ pathto('index') }}">{{ shorttitle }}</a>{{ reldelim1 }}</li>
+        <li><img src="{{ pathto('_static/py.png', 1) }}" alt=""
+                 style="vertical-align: middle; margin-top: -1px"/></li>
+        <li><a href="{{ pathto('index') }}">{{ shorttitle }}</a>{{ reldelim1 }}</li>
+{% endblock %}
+{% block extrahead %}
+    <link rel="shortcut icon" type="image/png" href="{{ pathto('_static/py.png', 1) }}" />
+{{ super() }}
 {% endblock %}