python-fuse vs python-fusepy on Debian hosts

Hi,

I've found the FUSE layer will break if the host happens to have both python-fuse and python-fusepy installed as the logic in fuse.py only expects ModuleNotFoundError and doesn't account for ImportError

https://github.com/wamdam/backy2/blob/master/src/backy2/fuse.py#L9

Unable to provide a PR at the moment though sorry