Fix Python 3 leftovers

This commit contains last small fixes. In Python 3 None cannot be
compared with int. Also string can be added only to string, thus
NamedUnicodeStr should be subclass of string.

Change-Id: I3afaa24cd88861e56b906689870ca7f1b8e853bb
Implements: blueprint py3k-support
Merge "Fix Python 3 leftovers"