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
4 jobs for !29 with topic/seda21/fix_tests in 9 minutes and 35 seconds (queued for 14 seconds)
latest detached
Status Job ID Name Coverage
  Lint
passed check-manifest #376560

00:00:29

passed flake8 #376559

00:00:42

passed yamllint #376561

00:00:26

 
  Tests
failed py3 #376562

00:08:53

 
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=376562 responseStatus=201 Created token=Wt4FU4tC
Uploading artifacts...
report.xml: found 1 matching files and directories

Uploading artifacts as "junit" to coordinator... ok
id=376562 responseStatus=201 Created token=Wt4FU4tC
Cleaning up file based variables
ERROR: Job failed: command terminated with exit code 1