chore: bump to 5.0.3, fix PyPI publish + Zenodo badge (#25)

- pyproject.toml + __init__.py: 5.0.0 -> 5.0.3 so the publish workflow
  can upload to PyPI (tags v5.0.1/v5.0.2 failed with 400 file already
  exists because the version string never moved).
- README: swap Zenodo DOI badge from zenodo.org/badge SVG to shields.io,
  which plays nicely with GitHub's camo proxy (the original rendered as
  a '?' on first hit).
- CITATION.cff: bump version to 5.0.3.
- CHANGELOG: document the bump and the badge swap.

Co-authored-by: JR Cruciani <jlrevilla@tutamail.com>