Support OpenAPI 3.1

**Describe the solution you'd like**

I have an [OpenAPI spec version 3.1](https://www.openapis.org/blog/2021/02/18/openapi-specification-3-1-released). 

I'd like to use hikaku with it, but when I do, it fails without a clear error message. I changed the OpenAPI spec to say it was version 3.0.3, and it works without issue.

