utils/types: implement __ne__ for level

This should have been handled by the @total_ordering decorator, but
isn't due to

        https://bugs.python.org/issue25732

(briefly, total_ordering is back-ported from Python 3, where the base
object provides the default implementation of __ne__ based on __eq__, so
total_ordering did not override it; this, however does not happen in
Python 2).

Also update unit tests to catch this edge case.

tests/test_utils.py

@@ -114,6 +114,8 @@ class TestEnumLevel(TestCase):
         assert_in('names', e.levels)
         assert_list_equal(e.names, ['names', 'one', 'two'])
         assert_equal(e.NAMES, 'names')
+        result = not (e.NAMES != 'names')
+        assert_true(result)
 
     def test_enum_behavior(self):
         e = enum(['one', 'two', 'three'])