zodb_pickle(): Use a lambda for convenience.

src/ZODB/tests/StorageTestBase.py

@@ -37,12 +37,9 @@ ZERO = '\0'*8
 
 def zodb_pickle(obj):
     """Create a pickle in the format expected by ZODB."""
-    def persistent_id(obj):
-        return getattr(obj, '_p_oid', None)
-
     f = StringIO()
     p = Pickler(f, 1)
-    p.persistent_id = persistent_id
+    p.persistent_id = lambda obj: getattr(obj, '_p_oid', None)
     klass = obj.__class__
     assert not hasattr(obj, '__getinitargs__'), "not ready for constructors"
     args = None