workshops/test/test_event.py: Use assertCountEqual for Python 3
workshops/test/test_event.py: Use assertCountEqual for Python 3

Python 2.7 has assertItemsEqual, but it's called assertCountEqual in
Python 3.2+ [1,2].

While adding the sys import, I also pulled the datetime import up into
the standard-library import block [3].

[1]: http://bugs.python.org/issue17866
[2]: https://docs.python.org/2/library/unittest.html#unittest.TestCase.assertItemsEqual
[3]: https://www.python.org/dev/peps/pep-0008/#imports

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