Provide more OpenAPI 3 example apps

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As of 2.0.0a2 rororo provides an example app to cover petstore-expanded schema from OpenAPI 3 Specification Examples.

Need to provide more examples (with tests) to ensure that most of cases are covered by rororo (via openapi-core).

Good schemas to cover:

• uspto.yaml
• hobotnica.yaml, which covers Hobotnica API (ref: https://igordavydenko.com/talks/ee-pycon-2019/)

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2.0.0rc0 added new example application: todobackend