Boolean schemas are incorrectly leaked backwards into previous drafts #523
Description
The relevant test cases here are these, which are currently marked skipped but should not be (they currently fail).
Probably we need to add a validator_functions_of argument to jsonschema.validators.create that, given a schema, returns an iterable of validator functions (which then permits booleans on Draft6/7 and not on earlier ones), but doing so is actually a bit tricky...