It hasn't been upgrading to Python 3 by itself but through the effort of a distro maintainer and other contributors: https://github.com/kovidgoyal/calibre/pulls?q=is%3Apr+author....

That is kinda the point of being an open source project, though, isn't it? The lead dev says, "It's going to be way too difficult for me to convert this codebase to Python 3, I don't want to do it." A group of volunteers then decide to undertake the effort themselves because they wish to ensure the project's continued viability for the future. AFAIK, he hasn't nixed the idea completely if others are handling the legwork.

Did he put any effort into the Python 3 port, or is his merit just that he didn't reject the PRs that others have worked on?

If you look at the github he is actively involved, and is fixing problems that the porters run in to.