If you run our test suite today with pytest four you'll notice that things fail. They've been warning for a while because we've been using pytest markers in old ways. If anyone wants to make a code contribution that doesn't involve much direct knowledge of Dask, switching our our tests to use markers correctly would be welcome.

You can easily recreate these errors by installing pytest==4 and then running our test suite. See http://docs.dask.org/en/latest/develop.html#install and http://docs.dask.org/en/latest/develop.html#run-tests just below.