Catch a known DeprecationWarning when calling .close()

[kurtmckee]

Pull Request check-list

Please make sure to review and check all of these items:

• Do tests and lints pass with this change?
• Do the CI tests pass with this change (enable it first in your forked repo and wait for the github action build to finish)?
• Is the new or changed code fully tested?
• Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?
• Is there an example added to the examples folder (if applicable)?
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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.

[kurtmckee]

Hello @gerzse and @vladvildanov! Please review this PR when you can. It's part of my effort to get CI tests to pass cleanly.