3scale Toolbox fails, it needs an "oauth" security scheme

Issue

When importing an Open API 3 specification for an anonymous API (no authentication) into 3scale using the 3scale Toolbox the Import OpenAPI step fails and the following error is written to the logs:

{
  "code": "E_3SCALE",
  "message": "OAS3 parsing error: oauth not found in security schemes",
  "class": "ThreeScaleToolbox::Error"
}