The failing test/test_demo.py tests are not be due to NFS but due to a different dbm backend in use. The file which it asserts should exist is _test_demo/db/nodes.user while on my machine where the test fails this files is named _test_demo/db/nodes.user.db
Note the extension .db

On the machine where this is working I have the debian package
python3-gdbm:amd64

while on the machine where this is failing this is not installed.
So the python builtin dbm support seems to chose different backends depending on what is installed.