chore: lint source code using Ruff

tests/utils/ast_node.py

@@ -12,13 +12,12 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-from typing import Union
 import ast
 
 __all__ = ['ast_node']
 
 
-def ast_node(statement: str) -> Union[ast.stmt, None]:
+def ast_node(statement: str) -> ast.stmt | None:
     """
     Returns an AST node from the statement.
 
@@ -26,7 +25,7 @@ def ast_node(statement: str) -> Union[ast.stmt, None]:
     :type statement: str
 
     :return: The node.
-    :rtype: Union[ast.stmt, None]
+    :rtype: ast.stmt | None
     """
 
     body = ast.parse(statement).body