the offlineimap3-git package assumes the source for offlineimap3 has a setup.py file.  the upstream has since moved to using pyproject.toml.
i made a patch to offlineimap3-git aur package that switches to building using pyproject.toml.  it now builds for me.

i'm hoping i can get the attention of the author, Pi-Yueh Chuan, to fix this in the AUR package.  i would post a comment to the package page, but i can't make an account on AUR because of the recent malicious updates.

also, i've never made a patch to AUR before, so it's plausible i'm doing it wrong.  if so, let me know!