tests: avoid assertNoLogs on Python 3.9

[dkropachev]

Summary

pyproject.toml declares requires-python = ">=3.9", but two unit tests on master use unittest.TestCase.assertNoLogs, which was added in Python 3.10 and is unavailable on Python 3.9.

Affected tests:

• tests/unit/test_response_future.py::ResponseFutureTests::test_set_result_no_metadata_statement_adopts_metadata_changed
• tests/unit/test_response_future.py::ResponseFutureTests::test_set_result_anomalous_metadata_id_warns_once_and_rearms

Observed behavior

Running these tests with Python 3.9.25 fails with:

AttributeError: 'ResponseFutureTests' object has no attribute 'assertNoLogs'

A direct runtime check also confirms:

hasattr(unittest.TestCase, "assertNoLogs")  # False on Python 3.9.25

Expected behavior

The unit tests should run on the minimum supported Python version. Replace assertNoLogs with a Python 3.9-compatible logging assertion/helper. Consider adding minimum-version test coverage to prevent similar regressions.