Add support for flask-openapi3

[githubjakob]

Really cool project, thank you!

It would be great to add support for flask-openapi3.

As far as I can see it should already work by adapting the check here https://github.com/BarnabasG/api-coverage/blob/main/src/pytest_api_cov/frameworks.py#L75 and use the normal FlaskAdapter.

[BarnabasG]

Thanks Jakob, this is a good find, thanks for using my project and for raising the PR, I have merged it.