virtnbdbackup: cannot import name 'CONTEXT_BASE_ALLOCATION' from nbd in Debian Bullseye about virtnbdbackup

[abbbi]

It seems, you have a python package "nbd" installed in /usr/local/ that is loaded instead of the python3-libnbd binding:

CONTEXT_BASE_ALLOCATION' from 'nbd' (/usr/local/lib/python3.9/dist-packages/nbd/__init__.py)

remove it or consider using a venv environment.

On my system, its having the CONTEXT_BASE_ALLOCATION setting with the nbd module
provided in /usr/lib by the python3-libnbd package:

~ $ dpkg -L python3-libnbd | grep nbd.py
/usr/lib/python3/dist-packages/nbd.py
abi@cefix ~ $ grep CONTEXT_BASE_ALLOCATION /usr/lib/python3/dist-packages/nbd.py
CONTEXT_BASE_ALLOCATION = "base:allocation"
    context, for example "CONTEXT_BASE_ALLOCATION".
    context, for example "CONTEXT_BASE_ALLOCATION".

[abbbi]

see comment -> environment issue.