made some optimization when parsing YAML files

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

This change makes a small speed optimization when loading a YAML file, but more important, it allows to use local stream wrappers instead of regular files on the filesystem.