Adjusted the arbitrary class used for persistent objects in a test to

deal with changes in hashability of persistent lists in Python
2.6. (It's still a puzzle why they are hashable in realier versions of
Python, but not in Python 2.6.)

src/persistent/wref.py

@@ -30,9 +30,9 @@ class WeakRef(object):
     Here's an example. We'll start by creating a persistent object and
     a refernce to it:
 
-    >>> import persistent.list
+    >>> import persistent, ZODB.tests.MinPO
     >>> import ZODB.tests.util
-    >>> ob = persistent.list.PersistentList()
+    >>> ob = ZODB.tests.MinPO.MinPO()
     >>> ref = WeakRef(ob)
     >>> ref() is ob
     True
@@ -47,7 +47,7 @@ class WeakRef(object):
     >>> WeakRef(ob) == ref
     True
 
-    >>> ob2 = persistent.list.PersistentList([1])
+    >>> ob2 = ZODB.tests.MinPO.MinPO(1)
     >>> WeakRef(ob2) == ref
     False