textproc/py-docstring-to-markdown: Update to 0.16

• Implement entry points to allow extension/customization
• Drop Python 3.7 from testing, add 3.12 and 3.13, update actions

Changelog: https://github.com/python-lsp/docstring-to-markdown/compare/v0.15...v0.16

Reported by: portscout, Repology