Add OpenAPI 3.2.0 support

[philfreo]

OpenAPI 3.2.0 was released.

This library currently supports 2.0, 3.0.x, and 3.1.x but not 3.2.x. Attempting to validate a spec with openapi: "3.2.0" raises OpenAPIVersionNotFound.

See also python-openapi/openapi-schema-validator#255

[philfreo]

https://github.com/python-openapi/openapi-schema-validator/releases/tag/0.7.1 was released, which should help this 🎉