Fix test failure in Python 3.5 and up

[StevenMaude]

Output for Python 3.5, but same test failure for Python 3.6, 3.7 and 3.8.

$ nosetests --exe

...................................F.

======================================================================

FAIL: test_empty (tests.TestUniqueKeys)

----------------------------------------------------------------------

Traceback (most recent call last):

  File "/home/travis/build/scraperwiki/scraperwiki-python/tests.py", line 140, in test_empty

    self.assertEqual(observed, {u'data': [], u'keys': []})

AssertionError: {'data': [], 'keys': ['seq', 'name', 'unique', 'origin', 'partial']} != {'data': [], 'keys': []}

- {'data': [], 'keys': ['seq', 'name', 'unique', 'origin', 'partial']}

+ {'data': [], 'keys': []}

----------------------------------------------------------------------

Ran 37 tests in 1.218s

FAILED (failures=1)

The command "nosetests --exe" exited with 1.