Make override work in <3.12

Author: mielvds

requirements-test.txt

@@ -1,3 +1,4 @@
 ruff
 pytest
 pytest-cov
+typing_extensions

tests/runtests.py

@@ -67,9 +67,10 @@
 # pytest now provides the test harness; these imports can be removed
 # once the legacy CLI runner is deleted.
 from argparse import ArgumentParser
-from typing import override
 from unittest import TextTestResult
 
+from typing_extensions import override
+
 sys.path.insert(0, os.path.join(os.path.dirname(__file__), '..', 'lib'))
 from pyld import iri_resolver, jsonld