Migrate from `setup.cfg` to `pyproject.toml`

[timobrembeck]

Motivation

To keep our tech stack as similar as possible across different projects (which makes it easier to allocate team resources between them), I suggest to migration from setup.cfg to pyproject.toml here as well similar to how we did it for the integreat-cms (see https://github.com/digitalfabrik/integreat-cms/issues/1362).

Proposed Solution

Follow the same steps as in https://github.com/digitalfabrik/integreat-cms/commit/303ba2cfabf6fd401e5038ce993ba05e9df7e9cf

Alternatives

Keep it as it is