revamping attrs-controlled class tests

[mattsb42-aws]

Bugfixes

• Remove use of attrs functionality deprecated in 17.3.0 attrs 17.3.0 breaks some of our tests #29

Maintenance

• Blacklist pytest 3.3.0 blacklist pytest 3.3.0 #32 pytest-dev/pytest#2957
• Add flake8-print
• Add docs build as part of default tox run and as a Travis CI build

Also currently blacklisting attrs 17.3.0. That may be changing, but I wanted to get this out for review while I consider what to do about that. python-attrs/attrs#303 (comment)

[lizroth]

These changes all seem sensible to me, but I don't think I have a complete understanding of the underlying issue and surrounding concerns. If you feel confident in this change, great; otherwise, you might want to pull in one more reviewer to double check me.