update util.base_url to handle python 3.9.17 change to urllib.parse.urlsplit

https://docs.python.org/release/3.9.17/whatsnew/changelog.html#changelog

> gh-102153: urllib.parse.urlsplit() now strips leading C0 control and space characters following the specification for URLs defined by WHATWG in response to CVE-2023-24329. Patch by Illia Volochii.

python/cpython#102153

Author: snarfed

util.py

@@ -586,7 +586,7 @@ def base_url(url):
   Args:
     url: string
   """
-  return urllib.parse.urljoin(url, ' ')[:-1] if url else None
+  return urllib.parse.urljoin(url, 'x')[:-1] if url else None
 
 
 def is_web(url):