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Description of change

When running the test suite, an expected DeprecationWarning is not caught. Recent example:

tests/test_asyncio/test_cluster.py::TestRedisClusterObj::test_close_is_aclose
  /home/runner/work/redis-py/redis-py/tests/test_asyncio/test_cluster.py:316: DeprecationWarning: Call to deprecated close. (Use aclose() instead) -- Deprecated since version 5.0.0.
    await cluster.close()

As this is expected behavior, this PR adds a pytest.warns() context manager to verify that this warning is issued and has an expected message.