Intern keys in `__setstate__`

Refs zopefoundation/persistent#15

See: http://blog.travis-ci.com/2014-12-17-faster-builds-with-container-based-infrastructure/

Also, work around edge case on Travis where files aren't found.

Exclude interfaces, whose method docstrings confuse the coverage checker.

Downloaded from https://bootstrap.pypa.io/bootstrap-buildout.py.

[ci skip]

The pure-Python Persistent class no longer calls subclass's __setattr__ at instance creation time, just like the C version.

KeyError would be raised during ZODB serialization; catch that. Not sure if this is the best solution or not but it gets zope.container passing.

If the python version of Persistent had __setstate__ called with a dictionary containing exactly two items, it would fail at idict.update(inst_dict) with 'ValueError: sequence of pairs expected' due to incorrect tuple unpacking. Do what the C version does and check for a tuple explicitly.

Fix traversal of the python picklecache ring when invalidating if the order doesn't exactly match the ring