Remove subscript notation for dict_items object.

Remove subscript notation for dict_items object.

In Python 3, a dictionary's items() method returns a dict_items rather
than a list type.  It's not possible to use subscript notation for a
dict_items type.  This patch fixes the issue, which was caught by the
following unit test:

[FAIL]
Traceback (most recent call last):
  File "/home/travis/build/NullHypothesis/bridgedb/bridgedb/test/test_main.py", line 159, in _eb_Failure
    self.fail(failure)
twisted.trial.unittest.FailTest: [Failure instance: Traceback: <class 'TypeError'>: 'dict_items' object is not subscriptable
/home/travis/virtualenv/python3.7.1/lib/python3.7/site-packages/twisted/internet/base.py:1292:mainLoop
/home/travis/virtualenv/python3.7.1/lib/python3.7/site-packages/twisted/internet/base.py:886:runUntilCurrent
/home/travis/virtualenv/python3.7.1/lib/python3.7/site-packages/twisted/internet/defer.py:460:callback
/home/travis/virtualenv/python3.7.1/lib/python3.7/site-packages/twisted/internet/defer.py:568:_startRunCallbacks
--- <exception caught here> ---
/home/travis/virtualenv/python3.7.1/lib/python3.7/site-packages/twisted/internet/defer.py:654:_runCallbacks
/home/travis/build/NullHypothesis/bridgedb/bridgedb/test/test_main.py:153:_cbCallUpdateBridgeHistory
/home/travis/build/NullHypothesis/bridgedb/bridgedb/test/test_main.py:55:mockUpdateBridgeHistory
]

bridgedb.test.test_main.BridgedbTests.test_main_updateBridgeHistory