Test expects ordered set on python3

[itziakos]

See in recent builds of traits-enaml (python 3.6).

======================================================================
FAIL: test_set_subscribe (traits_enaml.tests.test_trait_operators.TraitOperatorsTestCase)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/travis/.edm/envs/traits-enaml-3.6-pyqt4/lib/python3.6/site-packages/traits_enaml/tests/test_trait_operators.py", line 238, in test_set_subscribe
    self.assertEquals(enaml_widget.text, expected)
AssertionError: "TraitSetObject({'2', '1'})" != "TraitSetObject({'1', '2'})"
- TraitSetObject({'2', '1'})
?                  ^    ^
+ TraitSetObject({'1', '2'})
?                  ^    ^