tests fail with pytest 3.7.0+

I noticed that the tests for this project pass with pytest 3.6.4, but fail with 3.7.0 or higher.

https://gist.github.com/carlwgeorge/26a07a6c6b99cb6bfb3530668eafe5b9

I'm not sure what [changed in 3.7.0](https://docs.pytest.org/en/latest/changelog.html#pytest-3-7-0-2018-07-30) to cause this.  Travis appears to be preinstalling pytest 3.3.0, so it's not affected yet.