To me if the spec is clear and well defined this is a relatively trivial project (the test_this() side of it, and the docs side of it I don’t know - Andrew knows that side), so by now I trust you have all the input you need, and of course feel free to ask questions. Watching someone making small commits as they figure things out is not so fun. That’s why I suggested that I will be happy to look at the code when it’s more or less done, via PR or just name of your branch that you are working in.
Trust me, you won’t want to watch me code in real time, trying all kinds of things, most failing, so it’s nothing personal.
PRs are for ready things with potentially some fixes. They are not a development tool.
The dev way is to create a branch that several committers have access to and work on it together. No need for github’s pretty, but often not so useful interface.