[GNC] Have code run when a new invoice is posted.

[flywire]

I don't use that part of GnuCash but I couldn't imagine it being done
directly. Seems the options are either to rewrite the C++ code or process
it offline with a different workflow. For example,
https://bugs.gnucash.org/show_bug.cgi?id=798987 creates splits from bank
statements and https://github.com/flywire/ETF-Annual-Statement from tax
statements, both for csv import.

Given Python bindings are not available in a binary they are really a fake
feature and https://piecash.readthedocs.io is not genuine GnuCash so you'd
have to get support from that project.