Support build on Python 3.9.x

[sneumann]

Is your feature request related to a problem? Please describe.
I can't checkout and build https://github.com/ElixirTeSS/bioschemas-validator.git
on Ubuntu 22.04 running python 3.9.12 because

pip install -r requirements.txt
...
error in rdflib-jsonld setup command: use_2to3 is invalid.

The required change in RDFLib happened here:
RDFLib/rdflib-jsonld@2bf7fb8

Describe the solution you'd like
Bump the version in requirements.txt to e.g. rdflib==6.1.1
and remove rdflib-jsonld==0.5.0 since it is being "outroduced":
RDFLib/rdflib#1405

Additional context
There is a similar issue ckan/ckanext-dcat#205
and commits to fix: ckan/ckanext-dcat#213

I made the change locally and validation I need works, but I don't know if this breaks in a code path aI didn't test. I also don't know how to run the unit tests.

Yours,
Steffen