calibre-3.7 was released on the weekend: https://calibre-ebook.com/whats-new (portage is still at 3.3).
I'll see about making an ebuild for this new dependency: https://pypi.python.org/pypi/html5-parser
Version bumped to 3.8.0: https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=6d534217939125ad378a6ff013ddc6f39099d6fc