Couldn't such a tool exist outside the standard library. I'm thinking a function that you would import and wrap the parser object, similar to how argcomplete works (https://argcomplete.readthedocs.io/en/latest/index.html). The downside is that developers would have to opt-in for it to work (much like they currently have to opt-in to bash completion with things like argcomplete). But it would allow much more flexibility being outside the standard library.

I completely agree that it should be done in Python either way. help2man is very limited in its flexibility (it doesn't help that it's written in Perl), and there are fundamental limits to what you can do from parsing the --help output, vs. just generating correct troff from the source.

Installing the manpage is a separate concern. That would need to go in setuptools or distutils, if anywhere. But before you can worry about how to install it you need to be able to generate it in the first place.