Publish debusine to PyPI

It would be convenient to be able to install the debusine client from PyPI, when building on top of it.

At the moment, I'm having to install it from a zip file from salsa releases.

Ideally we would probably publish it as a separate Python distribution to the debusine server. Python Packaging doesn't make this straightforward... So I guess we'll just live with a single debusine package to PyPI?