update python packaging - get rid of pbr

apkg currently has one controversial dependency - pbr or Python Build Reasonableness (developed and used by OpenStack project) which I carried over because it provides automatic versioning functionality from git as well as some features making setup.py less painful. It was created to host packaging code duplicated across projects.

While the automatic development versioning from git repo is awesome and I don't understand why there isn't a PEP for that already, the rest of the library is superseded by modern python packaging PEPs and libs.

OTOH pbr introduces wide range of distro packaging issues including broken install when pbr is installed using pip as opposed to distro repos (apt install python3-pbr) on older Debian-based systems and I've seen it produce lousy warnings. It's also a runtime dep due to versioning functionality it provides.

After experimenting with modern python packaging in other projects I deem pbr's disadvantages far outweight what it brings to the table and I will get rid of it before %v0.1 in favor of modern solution using setuptools or poetry.

This is a prerequisite for #40 (closed)