Apistrap

- Migrate to OpenAPI 3
- Remove Flasgger and use apispec
- Add security decorator
- **BC break:** Remove autodoc decorator (now done implicitly, endpoints can be excluded with `ignore` decorator)
- **BC break:** Rename Swagger (the Flask extension) to `FlaskApistrap`

- Update `TupleType` to be a homogenous fixed-sized sequence
- Add `NonNanFloatType`
- Support `aiohttp` (`flask` is still supported)
- Support `StreamResponse`

Initial release. Main features:

- `accepts` decorator
- `responds_with` decorator
- error handling