bpo-31766: restore 3.5 to docs version switchers (#3971)

diff --git a/Doc/tools/static/switchers.js b/Doc/tools/static/switchers.js
index c2b342b..c450f5e 100644
--- a/Doc/tools/static/switchers.js
+++ b/Doc/tools/static/switchers.js
@@ -12,6 +12,7 @@
   var all_versions = {
     '3.7': 'dev (3.7)',
     '3.6': '3.6',
+    '3.5': '3.5',
     '2.7': '2.7',
   };