_asdict on ExtractResult empty on Python 3+

[otakucode]

When using tldextract on Python 3.4 I noticed that the dict attribute of the namedtuple ExtractResult would always be empty, which caused its _asdict method to also return an empty dict. This is because ExtractResult does not define a slots member, which prevents the parent classes dict from being added. I am not sure which version of Python changed this behavior. I added a unit test (and also added Py34 to the tox.ini environments) for this issue and it passed for Python 2.7, but failed for Python 3.4. Those are the only versions I had installed. I fixed the issue in tldextract.py and all tests now pass, both in 2.7 and 3.4. I created pull request #69 with the fix and additional test.

[john-kurkowski]

Merged #69. Thanks!

[john-kurkowski]

For posterity, this was a bug in Python, somewhere in the 3.4.x series. The PR's workaround is no longer necessary at least as of Python 3.5.2. See this SO discussion.