open api version 3 support

Hi,

I think that openapi version 3 support would be great for this tool. I am willing to help with this and create the necessary models that are different in version 3.

Would you be interested in this ?

The issue proposes OpenAPI version 3 support for this command-line tool but names no files, tests, or entry points. Start by locating how the tool currently models and compares OpenAPI specifications; done would mean version 3 specifications are supported without breaking existing comparisons.

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