Doc sidebar: 3.6 has moved to security-fix mode. (GH-11810)

(cherry picked from commit 9db56fb8)
Co-authored-by: Julien Palard <julien@palard.fr>

Doc/tools/templates/indexsidebar.html

@@ -4,7 +4,7 @@
 <ul>
   <li><a href="https://docs.python.org/3.8/">{% trans %}Python 3.8 (in development){% endtrans %}</a></li>
   <li><a href="https://docs.python.org/3.7/">{% trans %}Python 3.7 (stable){% endtrans %}</a></li>
-  <li><a href="https://docs.python.org/3.6/">{% trans %}Python 3.6 (stable){% endtrans %}</a></li>
+  <li><a href="https://docs.python.org/3.6/">{% trans %}Python 3.6 (security-fixes){% endtrans %}</a></li>
   <li><a href="https://docs.python.org/3.5/">{% trans %}Python 3.5 (security-fixes){% endtrans %}</a></li>
   <li><a href="https://docs.python.org/2.7/">{% trans %}Python 2.7 (stable){% endtrans %}</a></li>
   <li><a href="https://www.python.org/doc/versions/">{% trans %}All versions{% endtrans %}</a></li>