Test case:

In IDLE python shell:
>>> from http.client import HTTPConnection
>>> c = HTTPConnection(

Notice that the call tip is an empty parenthesis.

This patch works for me:

[/tmp/py3k/Lib/idlelib]
$ svn diff
Index: CallTips.py
===================================================================
--- CallTips.py (revision 78103)
+++ CallTips.py (working copy)
@@ -116,7 +116,7 @@
 def _find_constructor(class_ob):
     "Find the nearest __init__() in the class tree."
     try:
-        return class_ob.__init__.__func__
+        return class_ob.__init__
     except AttributeError:
         for base in class_ob.__bases__:
             init = _find_constructor(base)