First, when the docs say type=stream this means that you have to just pass "stream" for this parameter, which is specifying the type of entity to create. Also note the need for escaping JSON objects so they are treated as a single parameter. So in the command line it would be:
multichain-cli chain create stream mystream '{"restrict":"offchain,onchain,write,read"}'
In this Python library, I believe this would map to:
mychain.create('stream', 'mystream', {'restrict':'offchain,onchain,write,read'})
The Python dict is converted to a JSON object. Please let us know if this works for you.