add support for python 3 to remaining genshi.template components: * minor changes to track encoding=None API change in core genshi modules. * genshi/template/directives: * slightly odd syntax changes to make the 2to3 .next() fixer pick up *stream.next() * minor test fix for change in behaviour of division (/) in Python 3. * genshi/template/loader: * add 'b' to file modes to ensure it's loaded as bytes in Python 3. * use not isinstance(s, unicode) instead of isinstance(s, str) since the former is correctly converted by 2to3.