Response keys should be strings

[patrickkwang]

There are a handful of yaml schemas with something like:
https://github.com/zalando/connexion/blob/f31ac615bfc8a3c696c5ebe53f3c57e5234ff6f8/examples/openapi3/helloworld/openapi/helloworld-api.yaml#L15-L16
pyyaml interprets these keys as integers, while the OpenAPI specification requires that they be strings:
https://github.com/OAI/OpenAPI-Specification/blob/master/versions/3.0.2.md#format

The above should be replaced with

      responses:
        '200':

openapi-spec-validator does not validate this, but instead coerces int keys to str.

[dtkav]

I think the support for coercing int to str is from swagger2.