Since 45.0 readme_renderer uses comrak as Markdown backend instead of
cmarkgfm. comrak's Python bindings are not packaged in Debian and the
package still depends on python3-cmarkgfm, so
readme_renderer.markdown.render() always returns None (the Markdown
tests are silently skipped at build time).

This breaks reverse dependencies, e.g. postorius' autopkgtest
(test_list_info fails with a TypeError on None).

MR falling back to cmarkgfm when comrak is not available:
https://salsa.debian.org/python-team/packages/python-readme-renderer/-/merge_requests/2