python-ttystatus: remove python_versions; set to 2.7 by default
python-tracing: remove python_versions; set to 2.7 by default
python-larch: remove python_versions; set to 2.7 by default
python-cliapp: remove python_versions; set to 2.7 by default
obnam: remove python_versions; set to 2.7 by default