Both the django-mailman3 and the hyperkitty build failures look like the same issue , my looking however was focused on the hyperkitty failure as that is the one I became aware of first.
First I looked at test history for hyperkitty on reproducible builds and looking at when relavent packages changed. It seems that most-likely the change responsible for this failure was the upload of python3.8 3.8.4~rc1-1. Then I decided to try and write a minimal reproducer, the following seems to reproduce the issue. #!/usr/bin/python3 from email.message import EmailMessage from django_mailman3.lib.scrub import Scrubber msg=EmailMessage() msg.set_content('this is a test') msg["From"] = "sen...@example.com" msg.add_attachment(b'12345',maintype='text',subtype='plain') print(msg) scrubber = Scrubber(msg) print(scrubber.scrub()) I then decided to go have a look at the 3.8 branch of the cpython repository on github to see if there had been any recent changes to the email module and potentially to file a bug. However it seems it has already been fixed. Specifically it seems the issue was introuced in https://github.com/python/cpython/commit/c1f1ddf30a595c2bfa3c06e54fb03fa212cd28b5 And fixed in. https://github.com/python/cpython/commit/c1c50345933efca42169f03d79ff4fe3d9c06bdc Looking at https://www.python.org/dev/peps/pep-0569/ it looks like 3.8.4 final is due out today, I presume that will include the fix.