Throw an error if the user tries to parse an OpenAPI 3 document

[ffflorian]

No description provided.

[bennycode]

OpenAPI 3 documents have the following property in their JSON schema:

{"openapi":"3.0.0", ...}

Parsing the "openapi" property will help to detect if it is version 3 or below.

[bennycode]

Swagger documents don't have the "openapi" property. Instead those have markers like "swagger": "2.0". That's why checking for the pure existence of the "openapi" property is good enough.

I made a PR for that: #273