[Partial Fix] setup via pyproject.toml

Issue

As described in #249 (closed) this package is not pip-installable.

Description of work

These changes are sufficient to get the package installed. The installed package may need its requirements installed separately, so an improved solution would add the dependencies to pyproject.toml directly.

Checklist

• Pre-commit hooks have been run (see https://github.com/ess-dmsc/forwarder#developer-information)
• Changes have been documented in changes.md