Test expects ordered set on python3

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

```python
======================================================================
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'})
? ^ ^
```

Start with traits_enaml/tests/test_trait_operators.py and test_set_subscribe at the failing line shown in the traceback. Reproduce the Python 3.6 failure, inspect how the set value is rendered, and make the test pass without relying on set ordering; done means the test is stable across valid element orders.

Written by the indexing model from the issue text.