Enforce rdflib < 7.1 to prevent doctest breakage

This is only a workaround until these tests are properly fixed.
They should probably be converted as actual (py)tests; it would make it
easier to support these types of breakage due to formatting changes in
rdflib...

requirements.txt

@@ -7,7 +7,7 @@ frozendict != 2.1.2
 iso8601
 pybtex
 pyld
-rdflib
+rdflib < 7.1  # 7.1 breaks doctests; to remove when fixed
 sentry-sdk
 typing-extensions
 xmltodict