Doc: Make all versions sidebars the same for consistency. (GH-10288)

Doc/tools/templates/indexsidebar.html

 <h3>{% trans %}Download{% endtrans %}</h3>
 <p><a href="{{ pathto('download') }}">{% trans %}Download these documents{% endtrans %}</a></p>
-<h3>{% trans %}Docs for other versions{% endtrans %}</h3>
+<h3>{% trans %}Docs by version{% endtrans %}</h3>
 <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/3.5/">{% 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 %}Old versions{% endtrans %}</a></li>
+  <li><a href="https://www.python.org/doc/versions/">{% trans %}All versions{% endtrans %}</a></li>
 </ul>
 
 <h3>{% trans %}Other resources{% endtrans %}</h3>