CI breakage from pytest-cov 2.6.0

[njsmith]

Apparently the pytest-cov 2.6.0 release caused our CI to break: when we run coverage combine now, we get No data to combine and it errors out. Downgrading to pytest-cov 2.5.1 fixes things again (see #645), so that's the temporary workaround, but we should figure out how to get things working with 2.6.0.

This change is suspicious, since it touches coverage combine-related functionality: pytest-dev/pytest-cov#142

[njsmith]

Tentatively closing this since #647 seems to have worked, but hopefully we'll get a definitive answer from upstream here: pytest-dev/pytest-cov#222