Deal internally with OpenAPI 3 instead of Swagger 2.0

[u8sand]

Given that OpenAPI 3 has more features than swagger 2.0, any given swagger 2.0 file can be represented in OpenAPI 3 but not the other way around. In order to ensure feature completeness, it is therefore necessary to utilize OpenAPI 3 instead of Swagger 2.0 internally.

This may ultimately result in throwing away the pyswagger dependency which only works currently with Swagger 2.0