Doc: Python 3.10 in sidebar and version switcher. (GH-20209)

diff --git a/Doc/tools/static/switchers.js b/Doc/tools/static/switchers.js
index e1ef91a..c51f178 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 = {
-    '3.9': 'dev (3.9)',
+    '3.10': 'dev (3.10)',
+    '3.9': 'pre (3.9)',
     '3.8': '3.8',
     '3.7': '3.7',
     '3.6': '3.6',