CI: Fix typing after redis-py v5.0.0 release

[psrok1]

redis-py v5.0.0 adds support for asyncio. It affected return types from command methods which are now Union[Awaitable[T], T]. As the type doesn't really have sense (Awaitable types are returned only if we use Redis class from redis.asyncio package), it's difficult to pass returned values to other typed methods.

I have added temporary workaround for that by dropping the version to <5.0.0 for linting step (#226) but we need to fix it somehow.