Relax dependency on attrs

[DavidHuebner]

Is your feature request related to a problem? Please describe.
The setup.py currently requires the attrs package in version 21.2.*. This is outdated and we ran into conflicting versions e.g. with jsonschema that requires attrs>=22.2.0. jsonschema is e.g. required for jupyterlab, so some users have problems when they install the libraries sequentially and cannot currently use the newest version of jupyterlab.

Describe the solution you'd like
We should relax the dependency on attrs. In the 22.x and 23.x release the support for Python 3.5 and 3.6 was dropped, respectively. But this is not a blocker as cassis already requires Python>=3.7. To be on the save side, we can change the version to "attrs>=21.2,<24". I think that no changes in the code are required.

Describe alternatives you've considered
Not constraining the maximal attrs version may be problematic as attrs may not support Python 3.7 from version 24.x onwards.

Additional context
If you first run pip install jupyterlab and then pip install dkpro-cassis, pip will complain.