Don't assume Warning is an old-style class (i.e., in Python 2.5).

lib/python/Zope2/Startup/warnfilter.py

@@ -35,7 +35,7 @@ def warn_category(category):
             cat = getattr(m, klass)
         except AttributeError:
             raise ValueError("unknown warning category: %s" % `category`)
-    if (not isinstance(cat, types.ClassType) or
+    if (not isinstance(cat, type(Warning)) or
         not issubclass(cat, Warning)):
         raise ValueError("invalid warning category: %s" % `category`)
     return cat