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 !27 with topic/seda21/fix_tests in 9 minutes and 53 seconds (queued for 8 seconds)
latest detached
Status Job ID Name Coverage
  Lint
passed check-manifest #374906

00:00:23

passed flake8 #374905

00:00:45

 
  Tests
failed py3 #375059

00:09:07

failed py3 #374907

01:36:36

 
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=375059 responseStatus=201 Created token=zGnwczZP
Uploading artifacts...
report.xml: found 1 matching files and directories

Uploading artifacts as "junit" to coordinator... ok
id=375059 responseStatus=201 Created token=zGnwczZP
Cleaning up file based variables
ERROR: Job failed: command terminated with exit code 1