Fix 'url' parameter handling in KartonBackend.make_redis

[psrok1]

#292 introduced bug where url argument in [redis] configuration section doesn't work with rediss:// and unix:// URI schemes.

In #292, I've parsed the URL using redis.connection.parse_url to be able to strip credentials when Redis appears to not require authentication. That was bad decision because parse_url is meant for passing parameters to ConnectionPool class and not the Redis class. So e.g. when we use rediss scheme, we get connection_class parameter set to SSLConnection which is not accepted by the Redis client constructor.

So in this PR, I'm using Redis.from_url when url is passed in configuration. I'm also not trying to strip credentials from the URL - if someone relies on our the fallback for smooth transition, they must put the credentials in username and password configuration fields.

I have also noticed that we don't allow for password without username which is incorrect. Redis allows to set authentication both via requirepass option globally (which is not associated with any username) and ACLs (see also https://redis.io/docs/latest/commands/auth/). ACL system was introduced in Redis 6.0, so older Redis may reject the AUTH <username> <password> form. I have removed that requirement in this PR.