Comment 2 for bug 320149

Generally this error means you had a non-packaged version of pyyaml installed before and the package system declines to over-write admin installed files. From man pycentral:

       /etc/python/debian_config is searched for the attribute overwrite-local. If set to 1, or True, existing files in /usr/lib/pythonX.Y/site-pack‐
       ages are overwritten with symbolic links to the pycentral install area. The default is to abort the installation of a package, when files
       already exist. This behaviour may be useful, when python libraries were manually installed into this directory.

So you can over-ride this default.