Use jsonschema for validation

[patrickkwang]

A JSON version of the OpenAPI 3.0 specification is now available:
OAI/OpenAPI-Specification@6d17b63
One can validate against this using jsonschema instead of using openapi_spec_validator, here:
https://github.com/zalando/connexion/blob/master/connexion/spec.py#L227
In my experience, both work but jsonschema produces more useful error messages.

[rafaelcaricio]

Hi @patrickkwang,

This seems like an interesting idea since we would possibly be able to remove one dependency. Would you be interested in baking a PR with this change?