Compare py3 dev branch with reviewed code.

This MR was used to compare the dev branch with the review branch (which merged independent code changes as they were approved).

At the end of review it corresponded to changes made since review began, including some MRs reviewed in calls.

You can find the review MRs here: https://git.ligo.org/lscsoft/spiir/-/wikis/SPIIR-Review-O4b