While I was in here, I ported this code to use `pytestCheckHook`.
This fixes https://github.com/NixOS/nixpkgs/issues/211415 (maybe this is
more of a workaround?).
This build was failing due to a broken test upstream. I've instructed
nix to skip that test for now.
I also changed the `py.test` to plain `pytest`. I'm not familiar with
the difference, but according to [this SO
answer](https://stackoverflow.com/a/41893170/1739415), it sounds like
`pytest` is the new hotness and should be used wherever possible.
fd6c52733b