Introduce "poetry" for dependency management; Updates for Serlo & Science in School crawlers

[Criamos]

This PR includes:

• The introduction of poetry to the project (see: https://python-poetry.org), currently used for dependency management
  • this made updatess to pyproject.toml necessary
  • the updated requirements.txt was auto-generated by poetry (which also improves reproducibility during debugging/onboarding/project setup)
• all (but one) dependencies were updated to their latest version
  • fix: the pipeline for thumbnail processing used an outdated and deprecated method, which has finally been removed in 'Pillow' v10.0.0
• Updates for serlo_spider (v0.2.9):
  • highlight: by using the optional .env-Setting SERLO_INSTANCE="de" you can choose to only crawl learning objects from the German Serlo.org instance
• Updates for science_in_school_spider (v0.0.5):
  • fixes for unrecognized license edge-cases by removing the crawler-specific license mapping and instead using the LicenseMapper utility class
  • feat: author names are split up and saved to individual lifecycle-items