Fix flaky errors with py3

[dnephin]

We ran into some test failures while using jsonschema==2.4.0 with python3. The failures were intermittent (approximately 1 in every 10 runs), but were reproducible in both py33 and py34 (py26 and py27 were unaffected).

You can see the failures in travis-ci for this PR Yelp/swagger_spec_validator#11 or use that branch to reproduce them (tox -e py34). The best example is test_success in this build.

The code calling jsonschema is pretty straightforward:

resolver = RefResolver('file://{0}'.format(schema_path), schema)
jsonschema.validate(json_document, schema, resolver=resolver)

The error is:

Unresolvable JSON pointer: 'definitions/simpleTypes'

I added a bunch of debugging to jsonschema to see what was happening. From what I can tell, RefResolver.resolve_fragment() is being called with a ref of definitions/simpleTypes but the current base_uri in the RefResolver is actually http://swagger.io/v2/schema.json instead of what it should be (http://json-schema.org/draft-04/schema).

I also noticed that the ordering was totally undefined, every run would produce a different call stack. Some orderings would fail, and some would succeed, but there was definitely more than one order which caused it to fail. I tried a few things, and found that adding this sorted() to iterations before descend() resolved the problem. I tried with list() as well, but that didn't solve the problem.

Without being able to completely explain the problem, I can say that this fixes it. My understanding is that some combination of the undefined order of fields, and the lazy evaluation of iteritems() causes some call stack which results in the wrong state.

[dnephin]

It turns out that some part of #182 also fixes this bug

[dnephin]

replaced by the solution in #203