Wrong return type annotation in ProjectCommit.refs()

## Description of the problem, including code/CLI snippet

`ProjectCommit.refs()` returns `List[Dict[str, Any]]`, but the type annotations say it returns `Union[Dict[str, Any], requests.Response]`.

## Specifications

  - python-gitlab version: 3.0.0
  - API version you are using (v3/v4): v4
  - Gitlab server version (or gitlab.com):  -
