The version ofpytest on Travis's 3.6 environment seems to rely onsix>=1.10 so oursix==1.9 test config fails. See, e.g.,https://travis-ci.org/html5lib/html5lib-python/jobs/308631931

Given 1.10 is now over two years old, I'd hope we can drop support for 1.9, and fix this by just bumping our min required version to 1.10.