Test against Valkey

In light of many distros switching Reids -> Valkey, maybe we should test against Valkey too?

Or, maybe we should have a ValkeyConnection because eventually the two may diverge to a point where two different sets of APIs are needed.

I'd be happy to accept a PR for tests against Valkey. An entirely separate connection type feels like something that probably doesn't belong in this library, but we can see how things shake out.