Fix broken `Show Source` links on documentation pages (GH-3113) (GH-3125)

The `Show Source` was broken because of a change made in sphinx 1.5.1
In Sphinx 1.4.9, the sourcename was "index.txt".
In Sphinx 1.5.1+, it is now "index.rst.txt".
(cherry picked from commit b9ff4987)

Doc/tools/templates/customsourcelink.html

@@ -4,7 +4,7 @@
     <ul class="this-page-menu">
       <li><a href="{{ pathto('bugs') }}">{% trans %}Report a Bug{% endtrans %}</a></li>
       <li>
-        <a href="https://github.com/python/cpython/blob/{{ version }}/Doc/{{ sourcename|replace('txt', 'rst') }}"
+        <a href="https://github.com/python/cpython/blob/{{ version }}/Doc/{{ sourcename|replace('.rst.txt', '.rst') }}"
             rel="nofollow">{{ _('Show Source') }}
         </a>
       </li>