Fix Redis tox failures

[alexander-alderman-webb]

See the weekly run

https://github.com/getsentry/sentry-python/actions/runs/30634908199/job/91169923435?pr=6891

[linear-code]

PY-2640

[patrickswedish]

I’d like to take this. I reproduced the weekly failure and traced it to the Redis 8.1 ClusterPipeline constructor now dereferencing nodes_manager.himport_registry, while the test fixture constructs it with None. I also noticed the Redis tox environment is missing pytest-asyncio, so 26 async Redis tests are skipped rather than exercised. I’ll update the compatibility fixture and test dependencies, validate the supported Redis matrix, and open a focused PR.

[alexander-alderman-webb]

Feel free to open a PR @patrickswedish. I'd keep the scope limited to the Redis 8.1 failures. We can sort out the skipped async tests later.

We'll take it over if you don't get to it in the next day or so.