add support for python 3 to core genshi components (genshi.core, genshi.input and genshi.output): * default input and output encodings changed from UTF-8 to None (i.e. unicode strings) * Namespace and QName objects do not call stringrepr in __repr__ in Python 3 since repr() returns a unicode string there. * track changes to expat parser in Python 3 (mostly it accepts bytes instead of strings)