chore: limit rdflib to <6.0.0

rdflib 6.0.0 introduces a breaking change
serialize is a str and not a bytes object as in previous versions

--HG--
branch : seda21
6 jobs for !24 with branch/seda21 in 8 minutes and 25 seconds (queued for 1 minute and 6 seconds)
latest detached
Status Job ID Name Coverage
  Lint
passed check-manifest #376581

00:00:33

passed flake8 #376580

00:01:14

passed yamllint #376582

00:00:33

 
  Tests
failed py3 #376891

00:07:11

failed py3 #376780

01:25:15

failed py3 #376583

01:18:40

 
Name Stage Failure
failed
py3 Tests
Uploading artifacts for failed job
Uploading artifacts...
py3-deprecated-warnings.json: found 1 matching files and directories

Uploading artifacts as "archive" to coordinator... ok
id=376891 responseStatus=201 Created token=FmeQF6xL
Uploading artifacts...
report.xml: found 1 matching files and directories

Uploading artifacts as "junit" to coordinator... ok
id=376891 responseStatus=201 Created token=FmeQF6xL
Cleaning up file based variables
ERROR: Job failed: command terminated with exit code 1