Backport PR #6117: Remove / from route of TreeRedirectHandler.

When base_url is set, navigating to http://{url}/{base_url} returns 404

This is due to the TreeRedirectHandler only picking up http://{url}/{base_url}/ and the TrailingSlashHandler being set to {base_url}/.*/.

This change will cause http://{url}/{base_url} to correctly redirect to http://{url}/{base_url}/tree, as expected.
...

IPython/html/tree/handlers.py

@@ -99,5 +99,5 b' default_handlers = ['
     (r"/tree%s" % notebook_path_regex, TreeHandler),
     (r"/tree%s" % path_regex, TreeHandler),
     (r"/tree", TreeHandler),
-    (r"/", TreeRedirectHandler),
+    (r"", TreeRedirectHandler),
     ]