Quoting Cédric <boudi...@gmail.com>:
This is not a false requirement: you can try this: import pymodbus.client.syncI guess the package build succeeds because the dependency is only set in requirements.txt (for pip but ignored by setup.py) but not in setup.py (I think that this one is used by autopkgtest).
Thanks for clarifying this! I will add six to the dependencies.