Allow package to be pip-installed

[nils-werner]

While trying to fix an issue on macOS I noticed that I was unable to install this package using a pre-existing virtualenv and using

pip install -e rmfuse

I understand the attractiveness of Poetry, and do not want you to steer away from it. But I would suggest that this package should be installable in both Poetry and PIP.

[nils-werner]

OK, I don't think this is easily doable and I am comfortable enough in Poetry, so don't worry about it :-)