Installing ruamel.yaml creates two distinct packages: ruamel.yaml and ruamel.ordereddict.
Both are places under the ruamel root dit. However, since the ruamel root dir is missing the __init__.py module, importing those modules in python 2.7 is impossible (in python 3 the top level __init__.py is not mandatory and the imports still work). This issue is fixed via .pth files created in the installation dir. However, the installation process uses a custom dir (i.e. via --target or --prefix), the import process still fails.