Bug#1067620: add max version to python3-ruamel.yaml dependency

[David Mandelberg]

Package: whipper
Version: 0.10.0-2

From https://github.com/whipper-team/whipper/issues/605 and 
https://github.com/whipper-team/whipper/issues/606 it looks like whipper 
fails with newer version of ruamel.yaml. Should the Debian package add a 
max version to the dependency to prevent it breaking when 
python3-ruamel.yaml is updated?