Test failures with Python 3.3

3 tests sometimes fail (sometimes a subset of them). It is probably due to hash randomization.
(Test results from zope.schema trunk.)

$ PYTHONPATH="build/lib" python3.3 setup.py test
...
======================================================================
FAIL: test_ordering (zope.schema.tests.test_vocabulary.TreeVocabularyTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/tmp/zope.schema/build/lib/zope/schema/tests/test_vocabulary.py", line 274, in test_ordering
    ['2', '3'])
AssertionError: Lists differ: ['3', '2'] != ['2', '3']

First differing element 0:
3
2

- ['3', '2']
+ ['2', '3']

======================================================================
FAIL: test__validate_w_value_providing_schema_but_invalid_fields (zope.schema.tests.test__field.ObjectTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/tmp/zope.schema/build/lib/zope/schema/tests/test__field.py", line 1542, in test__validate_w_value_providing_schema_but_invalid_fields
    self.assertEqual(isinstance(err, RequiredMissing), True)
AssertionError: False != True

======================================================================
FAIL: test__validate_w_value_providing_schema_but_missing_fields (zope.schema.tests.test__field.ObjectTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/tmp/zope.schema/build/lib/zope/schema/tests/test__field.py", line 1518, in test__validate_w_value_providing_schema_but_missing_fields
    self.assertEqual("'foo'" in str(nested), True)
AssertionError: False != True

----------------------------------------------------------------------
Ran 398 tests in 0.283s

FAILED (failures=3)