Upgrade API spec to OpenAPI 3

[osma]

Annif uses Connexion to provide the REST API based on a spec file. The current spec file annif.yaml is written following the Swagger 2.0 specification. But this is quite obsolete, as its successor OpenAPI 3.0 was released already in 2017.

We should convert the spec file to OpenAPI 3.0 and probably also switch the name of the directory it resides in from swagger to openapi.

I already tried this, using Swagger Editor for conversion, but the converted version didn't seem to work - it gives a bunch of validation errors when I try to use annif run or run unit tests.