Fix use of override for Python < 3.12.

The `override` attribute is not available in the `typing` module in
Python 3.11 and lower, it needs to be obtained from `typing_extensions`
instead.

Author: gouttegd

src/sssom/rdf_internal.py

@@ -4,7 +4,7 @@
 
 import logging
 from datetime import date, datetime
-from typing import Any, Dict, List, Optional, Union, cast, override
+from typing import Any, Dict, List, Optional, Union, cast
 
 from curies import Converter
 from linkml_runtime.linkml_model.meta import PermissibleValue, SlotDefinition
@@ -19,6 +19,7 @@
     PredicateModifierEnum,
     SssomVersionEnum,
 )
+from typing_extensions import override
 
 from .constants import (
     ENTITY_TYPE_RDFS_LITERAL,