Intermittent failures in test_start_with_callback

[jaraco]

As observed in this job, some test runs will fail with

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FAIL: test_start_with_callback (magicbus.test.test_processbus.BusMethodTests)
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Traceback (most recent call last):
  File "/home/travis/build/cherrypy/magicbus/magicbus/test/test_processbus.py", line 273, in test_start_with_callback
    self.assertEqual(events, ['g', ('f', (1, 3, 5), {'foo': 'bar'})])
AssertionError: Lists differ: [('f', (1, 3, 5), {'foo': 'bar... != ['g', ('f', (1, 3, 5), {'foo':...

First differing element 0:
('f', (1, 3, 5), {'foo': 'bar'})
'g'

- [('f', (1, 3, 5), {'foo': 'bar'}), 'g']
?                                  -----

+ ['g', ('f', (1, 3, 5), {'foo': 'bar'})]
?  +++++
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Other times, the test passes just fine. I observed the same behavior on macOS. Likely there's some structure that's dependent on dict or set ordering.