Add test_files content to packaging
Provide test_files content in the distribution so downstream packagers sourcing from PyPI can run all tests successfully. While here, also note Python 3.6 support, since that's what I'm using it with.
Coverage remained the same at 100.0% when pulling f46f58955402ef967b3b8c9832ccba742e926013 on dhgutteridge:master into e96c00e31f3a52c01ef98193577d614d08a93285 on MartijnBraam:master.
Coverage remained the same at 100.0% when pulling 99099598b950ca95d64c2343373778261c3355a8 on dhgutteridge:master into e96c00e31f3a52c01ef98193577d614d08a93285 on MartijnBraam:master.
While packaging this for pkgsrc.org, which prefers sourcing distribution files via PyPI, I found the distribution file doesn't include the test_files directory, so six of the sixteen tests fail. It's preferable to include all test files for optional inclusion downstream.
One of the Travis CI jobs failed, but apparently not for reasons relating to this change. It looks like a fundamental problem with Python 3.3 coverage: "RuntimeError: Python 3.4 or later is required."