AppVeyor CI appears to be broken.

[abrisco]

Summary

I made what I expected to be a near noop change in #543 and ran into some failures:

• https://ci.appveyor.com/project/pytestbot/pytest-cov/builds/43991927

Looks like something is up with the windows environments?

[ionelmc]

Ref #547 .

[abrisco]

Seems as though #547 addresses this issue. Thanks @ionelmc! Closing the issue out.