[dagster-openai] Fix not called assertion in tests for Python 3.12 (#…

…26012)

## Summary & Motivation

`mock.not_called` is deprecated in Python 3.12, using `not mock.called`
instead.

## How I Tested These Changes

BK, local tests with Python 3.12

python_modules/libraries/dagster-openai/dagster_openai_tests/test_resources.py

@@ -71,7 +71,7 @@ def openai_op(openai_resource: OpenAIResource):
             )
 
         assert mock_client.called
-        assert mock_wrapper.not_called
+        assert not mock_wrapper.called
 
     result = wrap_op_in_graph_and_execute(
         openai_op,