What I did to solve this was to grab the setup.py from where it was removed:

https://github.com/pyhys/minimalmodbus/blob/b40e2946cd33684281dbf34ae1a8a4290cded649/setup.py

Then I updated it with the changes to setup.cfg (which disallowed old levels of Python).

By using this setup.py instead of my trivial setup.py my recipe worked just fine, and no longer attempted to do "import serial" which makes no sense on the build machine.

John Klug