> (2) src/calibre/ebooks/markdown/serializers.py license includes the text:

In fact, Debian deletes the markdown copy in the original tarball, but only
because they use their Debian markdown package instead. I looked at
python3-markdown, and the offending file is still in there.

$ guix import pypi Markdown
creates a package with bsd-3 license.

So I assume this file is in fact okay to use. A new patch is attached.

Andreas