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

diff --git a/Doc/tools/static/switchers.js b/Doc/tools/static/switchers.js
index 346b314..fa298a7 100644
--- a/Doc/tools/static/switchers.js
+++ b/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',