Sorry, I forgot to reopen the ticket while I was looking into the issue. Up to now I've found that the issue is an unclosed file ResourceWarning that only appears while building in sbuild (but not when manually running tests in the sbuild environment, nor while building in cowbuilder), and I'm still investigating where that comes from.
If anybody has any hints, they would be welcome. { message : ResourceWarning("unclosed file <_io.FileIO name='/build/reproducible-path/fpdf2-2.8.2/.pybuild/cpython3_3.13_fpdf2/build/test/errors/flowers.png' mode='rb' closefd=True>"), category : 'ResourceWarning', filename : '<frozen posixpath>', lineno : 72, line : None } See also the thread at https://lists.debian.org/debian-python/2025/01/msg00005.html -- Elena ``of Valhalla''