Test session with `nbmake` doesn't terminate on keyboard interrupt

**Describe the bug**
A running test session with e.g.`pytest --nbmake *.ipynb` cannot be terminated with `Ctrl-C`, the `nbmake` . The only way to terminate the test session was to kill the `pytest` process with `SIGKILL`.

**To Reproduce**
Run test session on multiple notebooks with a glob:

```bash
pytest --nbmake *.ipynb
```
Then try to terminate it with `Ctrl+C`.