I'm a Python noob myself.

1. No, the posting_key works ok like this.

2. It cant be

   for comment in steem.stream_comments()

I'm not trying to stream comments, but votes.

For that to work, some plugins needs to be enabled on the node.
They don't work on the default node ( at the moment )
Initialize Steem() with the "node=" argument, like I replied above.