Upgrade PyYAML and do not pin to specific version.
What's Changed
* chore: bump pyyaml 5.1 => 5.3.1 by gavindsouza in https://github.com/jonbeebe/frontmatter/pull/9
* Specify a loader when loading YAML by ColinOrr in https://github.com/jonbeebe/frontmatter/pull/10
New Contributors
* gavindsouza made their first contribution in https://github.com/jonbeebe/frontmatter/pull/9
* ColinOrr made their first contribution in https://github.com/jonbeebe/frontmatter/pull/10
**Full Changelog**: https://github.com/jonbeebe/frontmatter/compare/3.0.6...3.0.8