Doc: Python 3.9 in sidebar and version switcher. (GH-13824)

(cherry picked from commit 59e7bbca)
Co-authored-by: Julien Palard <julien@palard.fr>

Doc/tools/static/switchers.js

@@ -10,7 +10,8 @@
     '(?:release/\\d.\\d[\\x\\d\\.]*)'];
 
   var all_versions = {
-    '3.8': 'dev (3.8)',
+    '3.9': 'dev (3.9)',
+    '3.8': 'pre (3.8)',
     '3.7': '3.7',
     '3.6': '3.6',
     '3.5': '3.5',

Doc/tools/templates/indexsidebar.html

@@ -2,7 +2,8 @@
 <p><a href="{{ pathto('download') }}">{% trans %}Download these documents{% endtrans %}</a></p>
 <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/3.9/">{% trans %}Python 3.9 (in development){% endtrans %}</a></li>
+  <li><a href="https://docs.python.org/3.8/">{% trans %}Python 3.8 (pre-release){% 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 (security-fixes){% endtrans %}</a></li>
   <li><a href="https://docs.python.org/3.5/">{% trans %}Python 3.5 (security-fixes){% endtrans %}</a></li>