The tool should add a {{licensereview}} template to all uploads.

See more information about how to get the files on http://www.imslpforums.org/viewtopic.php?t=6853

See also https://commons.wikimedia.org/wiki/Commons:Gallica

It seems we don't need any code to get a HR image from Gallica. Gallica uses the IIIF protocol.
See https://medium.com/@seeksanusername/astuce1-r%C3%A9cup%C3%A9rer-de-la-hd-sur-gallica-bef0a6cc7f89 (in French) or http://gallica.bnf.fr/blog/24032016/apprenez-lire-les-url-de-gallica (in French).

If anyone wan't to hack on this but need's help finding where to start, whom to speak with or similar – starting this week there is now a weekly Technical Advice IRC meeting open for all volunteer developers: https://www.mediawiki.org/wiki/Technical_Advice_IRC_Meeting

Just that we don't duplicate work, this may be implemented in https://commons.wikimedia.org/wiki/Commons_talk:Glam2Commons#Gallica

Don’t see that this is pywikibot related because that script is currently not part of the framework.

A good first task is a self-contained, non-controversial task with a clear approach. It should be well-described with pointers to help a completely new contributor. Given the task description and the several options and links provided in comments here I'm removing the good first task tag.
Please add details what exactly has to happen where and how for a new contributor, and then add back the good first task project tag. Thanks a lot in advance.