Clean up monkey-patching of picklecache._SWEEPABLE_TYPES by the test case.

persistent/tests/test_picklecache.py

@@ -968,15 +968,14 @@ class PickleCacheTests(unittest.TestCase):
 
         import persistent.picklecache
         sweep_types = persistent.picklecache._SWEEPABLE_TYPES
-        try:
         persistent.picklecache._SWEEPABLE_TYPES = DummyPersistent
+        try:
             self.assertEqual(cache.full_sweep(), 0)
-
+        finally:
             persistent.picklecache._SWEEPABLE_TYPES = sweep_types
+
         del p._p_deactivate
         self.assertEqual(cache.full_sweep(), 1)
-        finally:
-            persistent.picklecache._SWEEPABLE_TYPES = sweep_types
 
     if _is_jython:
         def with_deterministic_gc(f):