Port to attrs version >=21.4

[hf-kklein]

There is a dependabot PR #297 which updates attrs to v21.4.
This version brings a lot of redability improvements of the attrs-API.

We should use these improvements here.

Namely the changes include:

• attrs.define instead of attr.s
• attrs.field instead of attr.ib
• attrs.validators instead of attr.validators

Todo:

In a fork of this repo:

• Switch to the new API
• Make sure all the CI actions still pass
• Open a PR to this repository

(You can check a similar change here).

[hf-kklein]

done in #378