Comment 10 for bug 353251

The fix is wrong because it doesn't consider the case the the relevant runtime not being contained in *any* category in /usr/share/python/debian_defaults.

Before: supported-versions was used as a whitelist
After: unsupported-versions is used as a blacklist

The current behaviour of python-central is to die if asked to install a runtime which is neither supported nor old - that behaviour needs to be considered here.