Unpin Pytest on Redis tests

We currently pin `pytest` for our Redis tests:

https://github.com/getsentry/sentry-python/blob/bf400904245c3809bad5f20fd637408f519e7a15/tox.ini#L585

Unpinning `pytest` causes many of the Redis tests to fail. See the status checks on https://github.com/getsentry/sentry-python/pull/3703 for details.

It seems that these test failures are occurring because data is leaking between tests. When running just one of the failing tests, the test passes.

Let's investigate what changed in Pytest in version 8.0.0 to cause this change in behavior, and see whether we can remove the version pin.