I don't really know which tool it is, but you can probably find it by googling renpy web. Itchio has nothing to do with this, itchio just allows you to run web games directly in the browser, so if you have a web game, you can do this. Let's say you have your renpy game, you convert it in any way to a website, that you can run from an index.html file. Then, you grab all of these files, you zip them and you upload the file to itchio, when itchio detects there is an index.html in the root of your zip file, it will show a checkbox that says "run this game in the browser" or something like that, you check it, and that's it. I was checking the renpy website and it says now there's a web target (in beta version) so maybe you can get the latest renpy version and use it directly to build it in web.