Took me a bit of Wiresharking to find this out, the problem is that we
are asking for the page using HTTP 1.1 in 3.0.

Here's a workaround patch for those who need it quick, I have yet to
look at urllib to see what can be fixed there.

---
Index: Lib/http/client.py
===================================================================
--- Lib/http/client.py  (revision 67716)
+++ Lib/http/client.py  (working copy)
@@ -600,7 +600,7 @@
class HTTPConnection:

_http_vsn = 11
-    _http_vsn_str = 'HTTP/1.1'
+    _http_vsn_str = 'HTTP/1.0'

response_class = HTTPResponse
default_port = HTTP_PORT
---


This is what we send in 2.5 and 3.0:

GET /cgi-bin/bugreport.cgi?mbox=yes;bug=123456 HTTP/1.0
               User-Agent: Python-urllib/1.17

GET /cgi-bin/bugreport.cgi?mbox=yes;bug=123456 HTTP/1.1
    Accept-Encoding: identity
    User-Agent: Python-urllib/3.1