Various minor OpenAPI fixes (#3375)

- In `create-class` for new schemas, use `dict` when classes cannot be created
- Do not create classes for properties starting with `_`
- Recurse into `$ref` when converting spec schema into Python type
- Make applying properties work with relative imports
- Fix spec loading for fetch command, flush progress bar
- Consolidate ignored schemas, schema inheritance, normalize class names
- Allow to decorate methods with returned schema property