Random order in RDFA’s root list

[Gallaecio]

In the current state of #127, running tox -e py -- README.rst and looking at the end of the reported diff between actual output and expectations, the order of the 2 dictionaries in the root list of rdfa changes across different executions.

So, sometimes you get:

       'rdfa': [ { '@id': 'https://www.optimizesmart.com/how-to-use-open-graph-protocol/#header',
                   'http://www.w3.org/1999/xhtml/vocab#role': [ { '@id': 'http://www.w3.org/1999/xhtml/vocab#banner'}]},
                 { '@id': 'https://www.optimizesmart.com/how-to-use-open-graph-protocol/',

While other times you get:

       'rdfa': [ { '@id': 'https://www.optimizesmart.com/how-to-use-open-graph-protocol/',
                   …
                   { '@id': 'https://www.optimizesmart.com/how-to-use-open-graph-protocol/#header',
                   'http://www.w3.org/1999/xhtml/vocab#role': [ { '@id': 'http://www.w3.org/1999/xhtml/vocab#banner'}]}]}

I though #116 would fix this, but it did not.