Update release

This release closes #15.

• Fix wrong version of pyyaml (^3.13 → ^5.3) in pyproject.toml
  • Also update poetry.lock
• Update version numbers (0.5.8 → 0.5.9) written in:
  • pyproject.toml
  • decode/__init__.py
  • docs/conf.py
  • tests/test_metadata.py