... and thus the report on coverage.io is empty.

I did two runs with identical source and almost identical .travis.yml where the only change is that I set a different python version for the test that runs the coverage results:

See these coverage results:
python 2: https://coveralls.io/jobs/12065924
python 3: https://coveralls.io/jobs/12063384

The important part of the Travis logs are here:
python 2: https://travis-ci.org/hamogu/astrospec/jobs/109594431#L536
python 3: https://travis-ci.org/hamogu/astrospec/jobs/109573956#L560