Replace `attrs` + `marshmallow` with `pydantic`

[hf-krechan]

The setup with attrs + marshmallow is not so easy to handle compared to pydantic.

At the moment the marshmallow-jsonschema needs setuptools as dependency which was a default package in the past.
But not anymore.
It seems to be insecure to add it into a package. I am not (yet) sure why.

We can streamline the dependencies by using just pydantic and it uses the same setup like in our other packages :)

[hf-kklein]

also using marshmallow-enum and marshmallow-jsonschema are more complicated than using pydantic, e.g. fuhrysteve/marshmallow-jsonschema#169

[hf-kklein]

the replacement won't be trivial, because of our own union type-implementation

[hf-krechan]

but nothing which our all stars team can not handle ;)