Migrated deprecated pkg_resource to importlib

[Aquaplant]

Hello,

I migrated the deprecated pkg_resources to importlib-standard. This required to update the Python interpreter to 3.9! This might clash with Issue 12

DeprecationWarning: pkg_resources is deprecated as an API. See https://setuptools.pypa.io/en/latest/kg_resources.html

For the sake of completeness: There are Keyerrors in multiple scripts caused by wrong type inputs either provided from the Doctests (and not by the migration); I will open an issue in an instance

Greetings, Fabian