ModuleNotFoundError: No module named 'usb'

When I try on my Mac (or under Linux):
```pymcuprog reboot-debugger```
and have an Atmel-ICE connected, I get the error message
```pymcuprog.pymcuprog - ERROR - Operation failed with ModuleNotFoundError: No module named 'usb'```
With an PICkit4 it works, though. 

More context: This only happens with the Python script installed using pip or pipx. The iot-provision binary executes without an error.


