Bug#973028: python-fabio's autopkg tests are failing with python3.9

[Matthias Klose]

Package: src:python-fabio
Version: 0.10.2+dfsg-2
Severity: serious
Tags: sid bullseye
User: debian-pyt...@lists.debian.org
Usertags: python3.9

see
https://ci.debian.net/data/autopkgtest/testing/amd64/p/python-fabio/7758092/log.gz