Thanks for your reply. I traced the problem to an error being triggered when looking up the dependencies (an instance of this problem: https://github.com/pallets/jinja/issues/655
). Since my own library code came later in the search, it was missed. I put in a quick fix on my own fork (https://github.com/alcrene/sumatra/commit/7b6d81d130b4e0c4f1eb368198866b3b0cbdcb36
) which works for me, although a proper solution (preventing any loading from looking up subsequent modules) might be tricky.
I do now confirm that libraries installed with `pip -e` are properly picked up by Sumatra. The software works beautifully.