For cases like this issue where there's not really much risk of unexpected interactions with other functionality, I wouldn't worry too much about it. Specifically in your case here, I'd probably just... check that os.startfile does the right thing for folders, text files and "other" files (which are the three clear cases that that code needs to handle) and that's enough.
While the instructions are based around GNOME tools, there's a guide at
https://wiki.gnome.org/Newcomers/SubmitContribution that covers the basic process. In short, make a fork on GNOME gitlab (where you already have an account), clone that locally, make your changes, push a branch with your change and create a merge request.
When you create the MR you can see what changes you're proposing, so that's a good time to double-check that there's nothing extraneous there. If you have issues or questions, I'm happy to try to help out. Also don't worry if the MR isn't perfect! It's all easy enough to change after the fact.
