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 aimaas.

Acceptance criteria

• aimaas's dependencies are managed by pdm.
• aimaas's container images are built using pdm.
• The documentation reflects this.
• The workflows are adjusted to take this into account, if necessary.