Python 3 compatible syntax: print, except, raise, octal numbers; removed...

... Python 2.2 boolean compatibility code in xlib/pydispatch/dispatcher.py

Tested using [check-python-syntax](https://github.com/alexanderlukanin13/check-python-syntax) with Python 2.6, 2.7, 3.3 and 3.4

With 3.2, it won't work that easily because of [u"" constants](http://docs.python.org/3/whatsnew/3.3.html).