This change replaces the creation of Redis URL, from a simple string
interpolation, to using `yarl.URL`. The main benefit, besides not
forgetting to set all five different variables on every Redis client
initialization, is that user credentials are correctly URL-encoded, if
present.
Up until now, if a password had special characters, it could break the
generated URL.
This change also introduces support for a `REDIS_SSL` setting, which
allows the user to specify if the Redis connection should use SSL or not.
Pytest v8.2 introduced the `PYTEST_VERSION` environment variable [1],
that can be used to check if code is running from within a pytest run.
This way, we can avoid checking the loaded `sys` modules.
[1] https://docs.pytest.org/en/stable/changelog.html#id57
Introduce an asynchronous Redis instance to be used in async functions.
Also, this change migrates most of the sync cache usage to the new async
cache.
