The ongoing python3 transition will pull out some needed dependencies for virt-manager and it will not be installable after du, at this moment.
EDIT: The issue comes from the new version of python3-chardet (4.0.0-1). Removing the new version, and installing version 3.0.4-7 from testing will allow virt-manager and virt-viewer to run after this morning's du, and doesn't appear to disturb anything else, at least on my systems.
Avoid this problem with # apt upgrade for this moment. Hopefully virt-viewer will get an upgrade to accommodate the new python3-chardet package.
EDIT 2: As of 1 JAN 2021, a new du resolved the conflict and python3-chardet is safely upgraded. Marking it solved.