Migrate to `pdm` for Python dependency management and packaging

[der-gabe]

User story

The devs of this project recently decided to move away from the old way of pip + requirements.txt for Python dependency management and setup.py for packaging to a new, modern, standards-based and fully integrated solution: pdm.

This issue is about accomplishing that for osc-tiny.

Acceptance criteria

• osc-tiny's dependencies are managed by pdm.
• osc-tiny is packaged using pdm:
  • for publication on PyPI and…
  • for distribution via Open Build Service.
• The documentation reflects this.
• The workflows are adjusted to take this into account.