workshops/test/__init__.py: Use an explicitly relative import

With the absolute_import future for Python 2 compatibility [1].  This
avoids errors like the following on Python 3:

  $ make test
  ...
  ======================================================================
  ERROR: workshops.test.test_event (unittest.loader.ModuleImportFailure)
  ----------------------------------------------------------------------
  Traceback (most recent call last):
    File "/usr/lib64/python3.4/unittest/case.py", line 58, in testPartExecutor
      yield
    File "/usr/lib64/python3.4/unittest/case.py", line 577, in run
      testMethod()
    File "/usr/lib64/python3.4/unittest/loader.py", line 32, in testFailure
      raise exception
  ImportError: Failed to import test module: workshops.test.test_event
  Traceback (most recent call last):
    File "/usr/lib64/python3.4/unittest/loader.py", line 312, in _find_tests
      module = self._get_module_from_name(name)
    File "/usr/lib64/python3.4/unittest/loader.py", line 290, in _get_module_from_name
      __import__(name)
    File "/.../amy/workshops/test/__init__.py", line 1, in <module>
      from test_event import *
  ImportError: No module named 'test_event'
  ...

[1]: https://docs.python.org/2/library/__future__.html
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
Merge pull request #67 from wking/test-relative-import

Get the test suite passing in Python 3