`tox` command fails at coverage report

When I ran `tox` command as described in [the document](https://pytest-cov.readthedocs.io/en/latest/contributing.html#development), the following error occured.

```
report: commands[0]> coverage report
No data to report.
report: exit 1 (0.13 seconds) /home/koyuki/pytest-cov> coverage report pid=112348
```